If you don't run out of 8 channels between your internal sounds and driving external gear - then MODX is very capable. MODX shines as a master. The zone control for controlling external gear is fairly flexible.

If you want your external gear to be full-keyboard controlled - and you want your internal tone engine to be triggered full-keyboard then, for that sound, you can "share" internal and external. When the splits are different between external and internal - you need to dedicate different PARTs for each. Your local keyboard keys only have simultaneous access to the first 8 PARTs - so that's a resource you'll either run out of or not depending on what you need.

If you threw out some specific scenario it'd be easier to explain why it'd work or not. But, in general, MODX is very capable as a master controller. Although there are some keyboards with more simultaneous zones - it's rare and MODX is at that 2nd tier (which is not bad). Even 8 zone, which MODX sports, is "rare" although less-so than 16 zone.

Ugh, I typed a reply and then it disappeared.
Let's try that again.

I would have about 3 or 4 synths connected to the midi out, each on a different channel, and I want to be able to easily switch between playing each of those, along with a few internal parts. It seems really simple, but everything I read seems to imply that to use the MODX like that I have to split the keyboard.

Hopefully that clarifies what I'd want to do with it.

Each MODX Performance HOME screen show you eight Part slots, divided into 1-8 and 9-16.
A Part slot can be an internal MODX sound or it could be an external device you are playing via MIDI.

You can play from one to eight of the slots directly with the MODX keyboard. Each MODX slot is equivalent to a Motif XF. so your MODX Performances internally can be using slots 1-8... for simultaneous play, if you so desire. You can setup your MODX sound to play simultaneously by activating a KBD CTRL (Keyboard Control) icon... literally when active (green) you controlling that Part with the MODX Keys.
If this icon is not active for a slot, by *selecting* that slot directly you are playing that instrument.

You could replace any of the slots with a Zone that transmits Out to one of your External synths.
So imagine you have a MODX doing a Piano from slot 1 (internally), slot 2 could be your first External Keyboard, slot 3 could be your second external keyboard, slot 4 is your third external keyboard, and slot 5 your fourth external synth...

By *selecting* 1 you are playing the piano internally, press 2 your playing that synth, press 3 and so on. Each is a separate synth, and you can seamlessly switch between these by simply tapping the Part slot.

You can in the next Performance, Layer any two or more together. By setting the KBD CTRL icon on more than one of the Part slots will allow you to control both from the MODX keyboard... this would be useful for splits, layers, complex combinations of splits and layers. Thevrule is you can control as many as eight zones simultaneously.

Yes, all eight could be internal, or you could have four Part slots combined playing internally, and four separate synth accessible via Part slots 5, 6, 7, and 8.
You could have eight internal , and setup your external synths as individually accessible Part slots, 9, 10, 11, and 12.

Not only can you access each independently, you can make virtually and combination of the five synths you can imagine within the maximum 8 Part slots simultaneously.

Any Part slot can be set to transmit Out on any MIDI channel, 1-16, as you may require.

It seems really simple, but everything I read seems to imply that to use the MODX like that I have to split the keyboard.

I was trying to see what would give you that impression, and I wonder if it was something like this sentence from the reference manual:

"You can divide the keyboard into a maximum of eight independent areas (called 'Zones')"

If that's the kind of statement that was throwing you, I'd point out a couple of things. One is that every Zone has upper and lower Note Limit parameters, you can assign them to whatever keys you want... including ALL of them. (Related to that, Zones can overlap in their key ranges, that's how you do layers rather than splits.) Also, a "maximum of eight" zones in no way prevents you from creating a "minimum of one" zone.

So to transmit on a single MIDI channel over the entire range of keys, just enable a single zone. You don't even have to alter anything to prevent it from splitting, because the default is for a zone to cover the full key range. To transmit on a second MIDI channel over the entire range of keys within the same Performance, just enable a second zone, Again, it will default to cover the full key range. As Bad Mister explained, you can switch between them with the KBD CTRL function. Alternatively, you can create multiple Performances that are each set to trigger just one of your external synths (i.e. each Performance has a single zone, set for the channel of a particular external device), and then you can switch among triggering various external MIDI devices by tapping different Performances from a Live Set screen. This may be the simplest way to go, at least if you're playing just one sound at a time (whether internal or external). That was something ambiguous in your posts... it is clear you only want to play one external sound a a time, but it is unclear as to whether playing the external sound would be in addition to simultaneously playing internal sounds, or instead of playing any internal sounds.