You can assign multiple instruments (staves) to one layout and hide the empty staves of the instrument not in use. So unless your back-and-forth is too rapid to use system breaks on the part to show only one staff at a time, that would be my first line of attack.

Thanks for the response. I'm unclear what you mean by "assign multiple instruments (staves) to one layout". Do you mean create 1 staff for each instrument, then hide the staves that aren't being used and combine them into a single part? Wouldn't that force the entire score to obey the system breaks required by that single group of staves? Otherwise you'd have two staves showing up in the score every time there's a change of instruments. It also makes parts very difficult to manage for that staff. Seems like just as much of a headache as creating my own MIDI map and Percussion Layout.

I was thinking of including both instruments in the full score but combing them in the player's part. You didn't say how many instruments were in your full score. In a large score, having the two instruments bracketed as a percussion group would not, I would think, be distracting. If the work is such that some performers are reading from the full score, then perhaps another solution would be more efficient.

You can hide empty staves in a Part without hiding them in the full score.

Generally, combining two instruments from different libraries into a single percussion map is difficult if not impossible.

It would make more sense to have all the parts in one staff, but I appreciate the suggestions. Combining two instruments from different libraries is actually incredibly easy - you just put each instrument in it's own layer or use an expression to change the Percussion MIDI Map. The only issue is that you can only change the MIDI Map, not the Percussion Layout, which means everything will playback just fine, but won't be notated correctly, as you can only set one Percussion Layout per staff.

… is it impossible to change the Percussion Layout for a single staff in the middle of a piece?  It seems like a pretty basic percussion notation need - I want my snare drum player to switch to a djembe or something. "Change Instrument" doesn't seem to work because I'm not using built-in Finale sounds, I'm using outside libraries …

… you can only set one Percussion Layout per staff …

I am only using the “built-in” Finale sounds, not any “outside libraries”, but I can change a single staff to another Percussion Layout “in the middle of a piece”.

As a matter of fact, in the dialog box Percussion Layout Selection I can choose between many Percussion Layouts.

What I do:

In the ScoreManager’s chiclet Instrument List the view is customized to show “Start Measure”.

In the column Name there is a triangle allowing me to expand  the view of the staff, and see all the separate measure regions with different Percussion Layouts or Percussion MIDI Maps.

* Can you do the same - with Finale’s “built-in” sounds? - with outside libraries?

I had no idea the changes made with Change Instrument showed up down there in the expanded view of the staff. This solves the problem perfectly. Thanks!