Do you mean switch from one solo Part to another solo Part?

All of your "solo" Parts can be in Parts 9-16 then touch the screen to select any one of these 8 Parts. They are, due to lack ability to group in a "Keyboard Control" group, by nature solo Parts.

Or do you mean to "solo" a group of Parts? Because say you have some sound that requires 2 Parts together to make up this sound and you want to "solo" the (multi-Part) sound? For this you would need to use keyboard control and place all Parts in 1-8. Use scenes to toggle the Keyboard Control ON for the "solo" sound Part(s) and OFF for all others. Your "on the fly" selection would be the scene buttons.

is it possible in a performance to select more than only one solo parts at the fly like mostof daws?

Technically, no. The SOLO Function works so that you can activate it on any one of the 16 possible Parts. Unlike a DAW (sequencer) where you are almost exclusively Soloing or Muting Tracks, the SOLO/MUTE function on the Keyboard is designed to work so that if you wish to listen to a select group of PARTs (multiple synth sounds), which are going to be triggered by pressing keys, you would do so by using the MUTE function to remove the ones you do not wish to hear.

SOLO is always going to be one Part isolated.
MUTE you can use as many MUTEs you require.

This type of SOLO/MUTE operation works for when programming sounds, while the multiple "solo" function works best for when playing back tracks, for example.
the MODX as a synth has an entirely different method of 'selecting' more than one Part on-the-fly... while performing.

Extra Credit: Isolation functions -- SOLO/MUTE status, SCENE Memory and Sequencer Playback
There are several uses for SOLO/MUTE in a music synthesizer like MODX. And there are several instances where its ability to ISOLATE what is sounding may be available via a completely different MODX routine. It will depend on the use case - and the operation that you are performing.

As you will discover the MUTE function disconnects the audio of the item from reaching the OUTPUT. It is like an ON/OFF switch. The light goes out, immediately. If it occurs in the middle of a musical note, too bad, it will be chopped off - like a light switch going out.

You will find there are several ways on the MODX that are music friendly when it comes to isolating or deactivating a Part. The KBD CTRL function allows you to discontinue a sound without cutting it OFF like a light switch. When you activate and deactivate PARTs using the KBD CTRL Memory Switch function, the sound will not be immediately cutoff... if you are in the middle of holding a chord, that chord will continue sounding until musically you tell it not to sound by releasing the keys or the sustain pedal that is causing it to continue.... All subsequent notes will play from the activated sound. So all transitions between what is sounding is "sonically invisible". No cutoff.

We mentioned when programming - being able to selectively isolate different components, temporarily, in an edit/compare, A/B type situation... each PART can have 8 oscillators, listening to the contribution of each to the total sound is one use case. In a KBD CTRL Performance each of the 8 linked Parts can contribute to the total sound, isolating these component needs to be as flexible, and as musically friendly, as possible. This is another use case.

Using Alternate Isolation Functions: If you are adding Parts to a Performance, for example, by using the "Performance Merge" function, you are given an option to compare the before and the after using the "Original" screen option and the "Add" option. This is a form of a Mute/Solo (isolation) function - very useful when combining two Multi Part programs into the same Performance. Before you drop in to the Performance you can preview the original alone, the components you are adding alone, and obviously both together. This takes place all on the "Performance Merge" Search screen.

Designed for performing, you will find the SCENE Memory function invaluable for this type of isolating of sounds.
Performance SCENEs (8) can be used to recall a specific set of Parts for interaction with the Keyboard (KBD CTRL) Parts. This can be used to determine which group of PARTS 1-8 you are currently controlling from the keyboard. There is also a way to create and automate what sounds are playing and which are not;

From the HOME screen, touch MIXING. This give you view of your internal digital mixer... you can view just Parts 1-8 plus the Audio Inputs (useful for working with KBD CTRL Performances), or you can view all internal synth Parts, 1-16 (useful for working with 16 Part multi-channeled sequences). There is a MUTE box near the top of each channel that can be used when playing back sequences - you have access to MUTE and SOLO soft buttons for each Track/Part. On the MONTAGE you have a hardware button for each Part for accessing the MUTE and SOLO functions... again SOLO will isolate one of the sixteen... while you can MUTE as many as you desire. On the MODX, you have a dedicated button for [PART SELECT_MUTE/SOLO] which provides a convenient overlay with your MUTE and SOLO buttons per PART.

If playing a linear MIDI SONG from the MODX sequencer, the SCENE buttons can recall Mute status as would be useful to create automation. Among the functions memorized by the SCENE MEMORY function is the Mute status - under "MIXING 2" - you can memorize what MODX Parts will be sounding, on demand. Say SCENE 1 has all tracks set to play. SCENE 2 could break it down to just the Bass and Drums, and so on (8 Scenes). These can be recalled while the sequencer plays back - you also have a way to SOLO a Part using the overlay.

It is possible to put several MODX programs in the same PERFORMANCE - then use the [SCENE] buttons or the Super Knob system to transition or morph between combinations of sounds. You mention "like DAW", but do not really outline if you are recording or performing

Necessarily a hardware synth has to have several ways to isolate sounds - for programming, for performing and for sequencing.
The main thing is to know that here, SOLO = one. MUTE you can have many
The current KBD CTRL condition can be memorized and recalled on-the-fly for performing.

Hope that helps.

grazie molte, mister! Si, era solo per poter ISOLARE per esempio BATTERIA E BASSO immediatamente, senza dover perdere tempo a mettere in mute altri 5-6 parti una dopo l'altra. E nel contempo, risparmiare sull'utilizzo delle varie scene. Per averne a disposizione per varianti più significative.

thank you very much, mister! Yes, it was just to be able to ISOLATE for example BATTERY AND BASS immediately, without having to waste time muting another 5-6 parts one after the other. And at the same time, save on the use of the various scenes. To have them available for more significant variants.

One Option you have is to use Control Assign to Mute all the Parts except the Parts you want to "Solo".

You can use an Assign Switch (AF1 or AF2) as a basic ON/OFF.... This will instantly cut off the Volume to the Parts you don't need. This may sound musically incorrect.

or

You can use a Single Common Assign Knob. For Arguments sake, we will say Assign Knob 1, but it could be any. Potentially, the Part Volumes can be quickly faded out, rather than cut off. It sounds more musical.

I recommend starting with an INIT AWM2 Performance just as an exercise in how to do this. An INIT Performance has NO existing control assigns. In reality, a Factory Preset that you have Stored as a User Performance will likely have many existing Control Assigns... including Part Volumes. In this case, you will need to re-design the Control Assigns, which is fairly complex, and beyond the scope of this example.

From an INIT Performance, Copy Part 1 to Part 2, Copy Part 2 to Part 3, Copy Part 3 to Part 4.
Change the First Element in Part 1 to a Strings Element
Change the First Element in Part 2 to a Brass Element
Change the First Element in Part 3 to a Woodwind Element
Change the First Element in Part 4 to a Syn Lead Element.

For this example, in a single "Motion":-

- you want to be able to MUTE Brass and Woodwind.
- you want to be able to SOLO Strings and Syn Lead.

Tutorial - Assign Knobs

Brass:- Edit Part 2. Edit - Part 2 - Common - Mod/Control - Control Assign
Click on the "+" screen icon.
In Destination 1, set "Part Param - Volume"
In Source, set AsgnKnob1
Curve Type = Standard
Polarity = Uni
Ratio = -64 (Negative 64)
Param 1 = 0, 1, 2, 3, 4 or 5. Changes Curve Shape. 0 = Instant Volume cut, 5 = Gradual Volume Fade

Woodwind:- Repeat the Process as per Part 2 above, except Edit Part 3.

Go to Performance Home. Click in the Performance Name window and select EDIT in Left Screen Pane.

EDIT - COMMON/AUDIO - Control - Control Assign

Click on "+" icon.
Set Source = AsgnKnob 1
Select Part 2 Assign 1 (Part 2 Assign Knob 1)
The Common (Performance) Assign Knob1 is now set to directly control Part 2 Assign Knob 1

Click on next "+" icon.
Set Source = AsgnKnob 1
Select Part 3 Assign 1 (Part 3 Assign Knob 1)
The Common (Performance) Assign Knob1 is now also set to directly control Part 3 Assign Knob 1.

For a professional job:- Click the "Display Name" window (top centre right). Type in P2&P3 MUTE.

Go back to Performance Home screen.

Note, now the first Assign Knob Control Window (top left of Performance/Part displays) is called "P2&P3 MUTE".

In order to Mute Part 2 & Part 3 turn Assign Knob 1 Clockwise.

ALTERNATE SOLUTION

Tutorial - Assign Switch

In order to use Assign Switch 2 (AF2) as a multi Part Mute

Brass:- Edit Part 2. Edit - Part 2 - Common - Mod/Control - Control Assign
Click on the "+" screen icon.
In Destination 1, set "Part Param - Volume"
In Source, set AsgnSw 2
Curve Type = Standard
Polarity = Uni
Ratio = -64 (Negative 64)
Param 1 = 0. Changes Curve Shape. 0 = Instant Volume cut. Other values have same result.

Woodwind:- Repeat the Process as per Part 2 above, except Edit Part 3.

There is no need for Common/Audio (Performance) Control Assigns in this case.

In order to Mute Parts 2 & 3 press the Assign 2 Button (Top Panel, Far Left, next to the Mod Wheel).

Extra Options:-

In the above Examples, you could similarly use Control Assigns to slightly Attenuate Volume Part 1 (Strings) and slightly Boost Volume Part 4 (Synth Lead).

As per the Assign Knob tutorial in previous post. These are "added" Destinations to Assign Knob 1 (Part and Common/Audio)

For Part 1 - Negative Ratio = -6, Curve = 5
For Part 4 - Positive Ratio = +6, Curve = 5

Add each of these to the Common/Audio Assign Knob1 as with Part 2 & 3.

You can set Correct Attenuate/Boost Levels for Part 1 and Part 4.

Turn Assign Knob 1 full Clockwise (maximum).

Edit Part 1 Control Assign. While playing the Keyboard, adjust the Ratio Parameter to find the desired level.
Edit Part 4 Control Assign. While playing the Keyboard, adjust the Ratio Parameter to find the desired level.

Turn Assign Knob 1 full counter-clockwise (to minimum). STORE the Performance.

Now, when you turn up Assign Knob 1:-
Part 2 will be muted
Part 3 will be muted
Part 1 will be attenuated (slight reduction in Volume)
Part 4 will be boosted (slight increase in Volume)

It should be noted that this means of volume-reduction-as-mute uses up polyphony, as it's not actually muting. It's merely playing at a 0 (zero) volume.

And it takes ages to setup, when Igor is looking for that rapid user input control technique of "solo addition".

I can think of no good reason why Yamaha hasn't used "solo addition" to empower more rapid isolation of Parts, operators and elements for performance consideration, usage, structuring and sound design.

If there is a good reason for it not being the default behaviour of solo selection setting, the way to add/provide this feature without interrupting/changing existing workflows is Shift+Solo to add to a "Solo-ing" of any part, element or operator.

This absence of solo-addition is far and away the most annoying aspect of sound design in FM-X.

Further, when exiting back up to Performance editing, soloing/muting of elements and operators is deactivated. This is extremely annoying.

thank you very much, mister! Yes, it was just to be able to ISOLATE for example BATTERY AND BASS immediately, without having to waste time muting another 5-6 parts one after the other. And at the same time, save on the use of the various scenes. To have them available for more significant variants.

grazie ancora!