You can play as many as eight Parts, simultaneously, using the MODX keyboard to trigger the MODX tone engine. The number of Parts you can play simultaneously from your external keyboard will be determined by the number of simultaneous zones it has... if it can Transmit on two zones it will be able to play two Parts of the MODX tone engine.

As an addendum to Bad Mister's reply, the zones on the external keyboard must transmit on different MIDI channels, (2) in your example. I had a similar scenario and thought that if I created a two sound split performance and then assigned that performance as a Part of another performance coinciding with the Part#/MIDI channel for the external keyboard, I would be able to play both sounds. Didn't work! If someone knows of a way to play a MODX split or layered sound from an external keyboard that transmits only on a single MIDI channel, whilst playing another split/layer sound on the actual MODX keyboard I would appreciate the answer!

There's a setting for that. Single Channel MIDI mode.

If each MODX Part is on a different MIDI channel, 1-16, how can you play those big Multi Part (8) Performances from a single channel Controller? The answer is, “MIDI I/O Mode” = Single. Found in [UTILITY] > “Settings” > “Advanced”, you can select a Single Channel on which to communicate to the MODX Tone Generator. This allows you to play the MODX sounds in the same fashion as if you were playing the MODX keys.

The other possibility is to leave MODX in multi-channel mode and place something between that will reroute the channels from a single channel to multiple. Camelot Pro does this. It's not the only option. There are computer/tablet or dedicated hardware (not computer based) that can do this.

@Richard

If someone knows of a way to play a MODX split or layered sound from an external keyboard that transmits only on a single MIDI channel, whilst playing another split/layer sound on the actual MODX keyboard I would appreciate the answer!

It is totally possible to play a Multi Part Performance internally (KBD CTRL active) while playing a split/layered Part from your external keyboard controller (no KBD CTRL). That answer appears below:

Set the MIDI I/O Mode = Multi.
A single Part can be made up of multiple instruments because each of the eight Elements of an AWM2 Part is the equivalent of a complete synth (Osc-Filter-Amplifier). Attatched below is an example. The MODX CONNECT .X8B File is zipped... unzip and use MODX CONNECT to bulk the program to your instrument’s Edit Buffer (it will overwrite nothing).

The “CFX Concert” is a four PART program played internally (KBD CTRL Parts active), and the StringBrassBell layer is split with an Acoustic Bass is played from your external controller (from Part 1). For the example, set the external Keyboard to transmit Out on Channel 1.
Parts 2, 3, 4 and 5 are the (internal) acoustic piano.

Part 1 (no KBD CTRL) is made from 6 Elements (shown below). It can be triggered by your EXTERNAL CONTROLLER transmitting IN on Channel 1 -- it is initially a 3 Element String Sound, raise the MW to bring the Brass Element, If you can activate AsSw1 (using cc86) to bring in the Orchestral Bell Element above C4, all of this layered for the right hand, plus the left hand split with an Acoustic Bass Element at F#2. This is all accomplished in PART 1, alone.

A single Part can be multiple instruments. Coming from the lineage of the Motif-series, there are tons of AWM2 data based on a four Element architecture, as found in the original Motif and Motif ES. All synths in the series feature single Elements that represent entire instruments, alone. Multiple Elements are typically used to add more nuance and/or more articulations to the sound during performance. These can be by way of Velocity switching, or by the various articulation controls. The trade off is that when it comes to assigning controllers, because all Elements are in the same Part, you may not have the ability to setup as flexible a control configuration. For example, they all will Pitch Bend together. But because much of the routing is on a per Element basis, it become a viable way to create multiple instrument sounds from a single Part. In the Motif XS/XF you had many "Voices" that combined instruments: "Piano & Strings", "Piano & Ooh Choir", "Ballad Stack", "80s Layer", 'Ballad Keys", "Strings & Choir", "Flute & Clari" etc., etc., etc

But depending on what you need from the sound, you can determine if the instruments can occupy the same Part or would benefit from having their own separate Part.

Those who dive deeper will find many possibilities by programming within the Part, at the Element level of the architecture (not just at the higher levels of the Performance).