The software tools would simplify manipulating Performances between two save files.

If the differences between your 7 and 8 are in the user area - this is where things have diverged - then the non-Melas way would be to choose one keyboard (7 or 8) to do most of the "shuffling" on. Lets say we choose the MODX7. You would first save your user area as a library file on the MODX7 (say you call it MODX7now.X8L) then install that same MODX7now.X8L library on the MODX7. You've effectively copied your user area to a library slot. Now you can (optionally) clear your MODX7's user area and still have the original data stored in the library area. On your MODX8 you can save the user data as a user file (say you call it MODX8user.X8U). Then take this file over to the MODX7 and load MODX8user.X8U onto the MODX7 - which would wipe the current user area on the MODX7 and replace it with the contents of the MODX8. Next, you can finalize the new combined set of Performances by selectively importing (use Library import) Performances from the "MODX7now" library (this is the "backup" of your original MODX7 content) to the user area. This is how you can merge the MODX7 specific Performances into the MODX8's. Once you are done and now your MODX7 user area has both the MODX7 and MODX8 Performances combined, you can remove the "MODX7now" library from the MODX7. Then save the user area as a user file (say you call it MODX78_all.X8U) and then, finally, install the combined user file on the MODX8 (load the MODX78_all.X8U file on MODX8).

Certainly at the beginning of this process you should either already have full backups of both keyboards or generate full backups of both. This covers if you make a mistake during any part of this process and inadvertently erase data which would otherwise have "lost" the data unless you had backups.

Note that the imported .wav data is still the same in both machines and the performances using the .wav data have not changed.

Then the process should be doable. Thank you for writing it out J, as it is more difficult to write out than to actual do it.

If the ROM Libraries are the same, then only the variations in User Banks needs to be dealt with. You current USER Bank data can be viewed and reviewed by pressing [UTILITY] > “Contents” > “Data Utility” > these Folders represent your current User data (the Library folder lists the installed ROM Libraries).

Using the Soundmondo app or MODX CONNECT to move individual Performances.
LIVE SETs can be a source of trouble... and since there is no one way to use Live Sets, this could be easy or very complex.

Here’s what I mean... a Live Set can reference, “point to”, any Performance, be it a Preset, a User or a Library program.
And this is where the help of the Melas Tools can be helpful. You can work completely offline, and create a file that can be loaded to both instruments.

If you have created new Arps, Patterns, Songs, Curves, Motion Sequences, etc.... you must ensure these are identical in each unit.

I dare not speculate on how you use the Live Set - it a feature that is really up to the individual to use as they may require.
You may keep specific Live Sets in your Library installs; you may make a new one for each gig; there is truly no single way to use this feature.

If you create a Pattern or Song in one of the instrument’s, you can move them via standard MIDI File (.mid) — use a USB stick for this.

I think the analogy used before is that driving takes a lot of steps if you really write down everything involved in just driving to your local store. And most all drivers on their second year would say it's "easy" even though it takes a lot to write down. Because eventually these steps become second nature and the length of performing some of the steps is negligible.

Still, it would be a great long-term plan for Yamaha to pursuit elegance in the user interface. Where operations take less steps and the user can approach the system in a more intuitive way without as much need for the manual or forum assistance. It can be done and there are popular leaders in user interfaces that demonstrate if user experience is a focus - then product loyalty can be driven by elegance even when the hardware specs are inferior to competing products.

On Live sets - there's not a merge feature with Live Set. Nor is there a way to extract just the Live Set apart from the rest of the data in any user area. Therefore, Live Set on the keyboard itself is mostly relegated to hand editing from the ground-up (concerning the new Performances) after you merge (import) content from the Library to the User area. Depending on how many Live Set entries you would have to add manually (again, still assuming just using the keyboard itself - not software assist) then this may dictate which keyboard's content is the Library area merged in (and therefore any Live Set pointers would be missing for this content in the merged data) vs. the user content (saved X8U file during shuffle) which would include all of the Live Set pointers for those Performances. My steps basically said the choice is arbitrary - but this is one consideration to guide the choice should Live Set be something you use.