You can combine MONTAGE Performances into one Library in the MONTAGE itself. Here is the theory of the operation.

Data in your MONTAGE Library area can be “imported” to the MONTAGE User Bank, using the “Library Import” feature.
[UTILITY] > “Contents” > “Library Import”
A MONTAGE Library is created by saving the data you assemble in the User Bank area of the instrument... you then, literally, “save as...” a Library File. You would take data from your Libraries and assemble them in the User Bank...then create a new Library File.

The USER Bank can hold the following data maximums:
640 Performances
2,048 Waveforms
2,048 Live Set slots
256 Arpeggios
256 Motion Sequences
32 Curves
8 MicroTunings

When you move data from a MONTAGE Library to the MONTAGE User Bank, using the “Library Import” Function, the Data is redirected so that it is utilizing User Bank Performance locations and User Bank Waveform slots, and so on.... this “Importing” Function does not Overwrite data in the User Bank, it simply adds your selections to the lowest numbered empty locations in the USER Bank.

The MONTAGE will keep track of how many Performances out of the 640 you have filled.
The maximum amount is based on reaching any of the limits... yes, you can have 2,048 Waveforms but there still is the 1.75GB overall limit.
There is also an individual number of samples limit (8,192 samples). If you reach any of these limits the memory will report full. Even if you have not reached the 2,048 Waveform limit.

What that means is: 256 Samples can make up a Waveform. Say you reach 8,192 individual Samples before you reach the 2,048 Waveforms, you will get a memory full warning. Say you reach the 1.75GB limit before you get to 2048 Waveforms, you will get a memory full warning. So whichever Limit is reached first causes “memory full”.

Once you have assembled the data from your Libraries into the User Bank areas... you can DELETE that original Library (Libraries).
Then SAVE/STORE a new .X7L file to a USB stick using the “Contents Type” = Library File.

Once you have created your combined .X7L File.... Initialize your User Bank, then install your newly made Library to your MONTAGE.

SUMMARY:
The Library area is ROM (Read Only Memory) — this type of memory is the expensive kind that allows you real-time access to this data with response times in the nanosecond range... it can deliver 128 Stereo Notes of polyphony immediately. (The type of memory you find in storage devices like a thumb drive (relatively inexpensive) has a much, much slower access time and can only deliver a single Stereo stream — making it good enough to “playback” a stereo recording. But it could not dream of delivering 128 Notes of Stereo polyphony in the “on demand” requirements it takes to play music in real-time.

Preparing Libraries is time consuming... this is because you are in effect, “burning” the Wave data to ROM.
The data you burn/install to a Library will remain in your instrument until you either DELETE it, or INITIALIZE ALL DATA.

Extra Credit:
The John Melas MONTAGE Waveform Editor is the most elegant way to manage the creation of MONTAGE Library data. This is a third party program (written by a long time Motif/Montage User from Greece) and is simply a brilliant piece of work.
It allows you to do the heavy lifting (moving data and repointing Element data) on your computer — where you have more room to work and you can really speed the process of Library consolidation (and much more... see link below to see a detailed description).

Learning to this on the MONTAGE itself, is totally possible. And recommended so that you can understand the process. But if you plan on creating your own custom Library data... the Waveform Editor is the tool you will wish existed. Thankfully it does... after having assembled Libraries manually — as I described above — you will really, really appreciate the speed, and ease of assembling your data with a tool like the Melas Waveform Editor.
Melas MONTAGE/MODX Waveform Editor

Thank you for the details @Bad Mister, I really appreciated.