Yes, you can achieve this effect with ANY arpeggio, by changing the timings on the common page of arpeggios:

the cryptically named: "Arp/MS Grid" button sets a minimum spacing between notes of an arpeggio's playback. It's of no use without the value next to it "Qntz Strength" Which means the strength of quantisation applied to the notes of the arpeggio such that they respect the arp's grid from the box to the left, which doesn't seem immediately useful, or linked... another foible of the UI designers...

If you set that next button's value "Qntz Strength" to 100%, then arp notes will be pushed to the grid lines of the "Arp/MS Grid" setting. At 120 this isn't likely to do much as that's fairly granular. But if you set this to 240 or 480 you'll be stacking all the notes that occurred during that time onto the next grid line at this resolution. A kind of chord.

The "Unit" value directly below is another wonderful misnomer. This is the rate the arpeggio plays back relative to the current tempo. Unfortunately this doesn't go any faster than 50%... yes, you read that right... 50% is faster. Yamaha's designers don't do design for humans, they merely lightly skin what the programmers tell them is there.

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There's also a bunch of arps that play chords. Mostly in the EDM type of ranges. I can't remember what they're called or what they have as an identifier that shows they play chords (probably nothing this useful).

And you can use the gated arps to play chords as they merely take whatever you play and "gate" the audio signal in various ways to make a kind of chord pulsing sound.

Suggestions to create Polyphonic Harmonies

1) 2 Part Split, with same Arpeggio in each Part. Play lower Harmony (Chord) with Left Hand, and upper harmony (Chord Extensions or Inversions) with right hand. You obviously need to be an adept pianist and know your theory.

2) Set up fixed Intervals (e.g. 4ths, 5ths, min3rd+5, maj3+5 etc) with Element or Operator/Carrier tunings. Playing a single key will sound as a Chord/Harmony. The MODX cannot be configured to recognise, or play in a specific Musical Key or Mode (Eg C Dorian, Bb Melodic Minor etc). Therefore harmonies will always be at set pitch intervals. I.e. there is no "intellgent harmonizer" function.

Common Unit multiply means the Unit Multiply assigned in the Common/Audio section of the Performance. This is a global value that you can use to control each Part with a single common change.

1) Press [PERFORMANCE] (HOME) button
2) Touch the Performance name at the top
3) Choose "Edit" on touchscreen (TS)
4) TS: select "Motion Seq" -> "Common" menu
5) The second circular (at least on Montage) "dial" from the left is labeled "Unit". This is Unit Multiply for the common area.
Going further ...
6) If you press the [ARP/MS] button (only available on Montage since it has 8 knobs in front of you for 8 settings vs 4 on MODX) which is to the left of the Assignable Knobs (and a little towards the back of the instrument or "up" ) then there's a "Unit Multiply" knob which is Knob #2 (from the left). Turning it will show the "Unit" dial move around in sync with the knob movement.
7) You can also see Common Unit Multiply with ... first pressing [PERFORMANCE] (HOME) again
8) Then TS: touch "Motion Control" which is a menu on the left side.
9) Navigate to menu "Motion Control" -> "Quick Edit"
10) If the right-hand side box says "Part / Common" then look at the bottom row of dials. The 2nd from the left is the "Unit" dial which again is Common Unit Multiply. For Montage users, they can turn the same Unit Multiply knob (as long as [ARP/MS] fixed function button is illuminated) and the "Unit" dial on the touchscreen will move in tandem.
.... and
11) Under [PERFORMANCE] (HOME) then "Scene" - the "Arp/MS FX1" tab (Turn on these ARP/MS FXs with the memory ON button) you will see that FX1 has "Unit". This doesn't change the Common Unit Multiply.

[quotePost id=119240]Common Unit multiply means the Unit Multiply assigned in the Common/Audio section of the Performance. This is a global value that you can use to control each Part with a single common change.

[/quotePost]

This is opaque to anyone that doesn't both know what this means and know the terminology. I know the terminology, but that's not enough to unpack these two sentences into meaning.

Can you explain it?

Not demonstrate it, not give an example, not a tutorial, actually explain what each is, what can be done, and why these things are things?

Common Unit Multiply is a means to link the common (global) level to the Parts. The Common Unit Multiply is a "Super Knob" of sorts for the Part level Unit Multiplies linking the two (like superknob link) when you fill in "common" for the Part level Unit Multipliers.

The previous question was where to find Common Unit Multiply so that's what I answered.

You know what Unit Multiply is - so I won't rehash that.

Whenever you see "common" as a choice for a Unit Multiply it means to take the Common Unit Multiply value and stick it in the given location (Part 1-16's Unit Multiply, Scene). When the Common Unit Multiply value changes, all of the other places where you filled in "Common" for Unit Multiply get replaced with the new Common Unit Multiply value.

An application of this would be to setup all Parts as "Common" for unit multiply then the Common Unit Multiply could be changed from 100% to 50% to double time all the Parts since the Common setting flows through.

I don't know the why. Why is the sky blue? I mean ... I don't want a tutorial about the atmosphere. Philosophically conditions could have been different and the sky could be any color. Why did destiny pick blue? I don't know. We'd all be the wrong authority on any of that. All I can do is speculate and know that common unit multiply was a thing in past generations' synths each with different levels of macro assignments so with such scarce resources this may have been added in order to allow for a certain style of macro control.

Back to the original question - Andrew presented a good "trick" to force some polyphony out of a melodic arpeggio. This does impact the placement of the notes so that may be good or bad for your goals.

There isn't a button or setting you can press/modify that would turn a melodic arpeggio into a harmonic one. Say that lets you define a minimum/maximum number of notes to play so you can take a "1" (monophonic) and tell it to play triads. Or tell it to play as many notes as you hold down.

However ...

There would typically be a harmonic version of the same arpeggios available. Speaking broadly, there's lots of "Rock" arpeggios. The Bass Guitar category "Rock" arpeggios are mostly melodic because typically the bass is played this way. There would typically be "matching" arpeggios in the Guitar or Piano (or other monophonic instruments) categories that would provide the choice of a now polyphonic arpeggio.

The way to get a more "exact" version of the original melodic arpeggio as harmonic would be to pick the "right" (different) preset arpeggio.

The other choice if this exactness is what you're after is to create a user arpeggio. You cannot create every possible Arpeggio type but you can create ones that are harmonic. And those harmonic User Arpeggios can either transpose as you move up/down the keyboard, stay fixed, or pass through your fingered notes as-is and just apply a rhythmic pattern to those notes.