The MX88 is 16 Part Multi-timbral for use with an external Sequencer. From the front panel you can play any one of those sounds by pressing [PART SELECT] then using buttons [1]~[16] to play the corresponding instrument assigned to a Part.

From the front panel you see the markings that indicate Part 1 (solid bar) top line of the screen, and Part 2 (open bar) second line of the screen.
The [LAYER] button indicates the solid bar atop the open bar across all keys. The [SPLIT] button indicates the solid bar on the right side the open bar on the left. Without much difficulty you can guess that Parts 1 and 2 can be combined in either a Layer or a Split.

When the [SELECT] button in the middle is not lit, you see the Category and Name of the currently select sounds in Part 1 top, and Part 2 bottom. Use the CURSOR UP (solid) and CURSOR DOWN (open) to move between the two sounds. A small keyboard icon appears next to Part 1 and/or Part 2, when you touch a key and that Part is sounding.

From the front panel that is the maximum number of sounds you can play simultaneously directly with the Keys. When Part 10 is set to a drum kit, you can use the [RHYTHM PATT] button to play a drum groove in responds to touching a key.

Layering more than one side of a split with Voices is possible but only via System Exclusive (Parameter Change) messages. One of the powerful things about the MX line is that it features many of the Voices found in its bigger siblings (the MOXF and Motif XF/XS)... It shares much of the same synthesis engine. They did not skimp on the Synth engine or sound... but you do not get the same depth of editing from the instrument’s front panel.

For those interested in digging deeper into editing (far beyond the MX88 front panel) we can highly recommend the John Melas MX TOOLS (which includes a Librarian, Voice Editor, Performance Editor, Drum Kit Editor) and truly unlocks the deeper levels of the MX. John Melas is a third party programmer who created these powerful editing tools that allow you get at the deeper parts of the MX synthesis engine. The DATA LIST booklet for MX49/61/88 Contains all the Parameter Change messages to edit this deeper level of the architecture... Mr. Melas has dug in and created a useful set of tools for deep diving.

Alternatives:
When the first version of the MX49/MX61 came out five years ago, there was a free Performance Editor ...”VycroMX Performance Editor” (I don’t know if the company is still around...) but the Performance Editor was free and gave some bit of editing, but not near as thorough or deep as the Melas Editors).

Programming with the VycroMX Performance Editor

We’ve posted an MX tutorial article on “Synthesizer Basics” that uses the John Melas MX Tools for the examples. You might want to read through it to get an idea of what’s available.
MX Study: Synthesizer Basics with the MX