..are actually used in existing MSX software? Either games, music editors, demo's, test software, whatever.
For example SCC + PSG should be very common. MSX-Music + MSX-Audio also. That, +PSG? Probably. But what about more exotic combinations?
Say, MSX-Music + Moonsound. Or SCC + PSG + MSX-Music. Or test software that exercises all soundchips known to MSXkind all at once? What combinations do you know of, and what software does it? I don't mean where you can choose between X alternatives, but rather where they're used at the same time (mixed). Or ones that require multiple instances of chips (eg. 2x YM2413) by way of hardware support or emulation.
With the plethora of soundchips used on MSX, there should be quite a selection in this regard I suspect. Unless programmers tend to stick to 1 or 2 preferred chips, and using more / uncommon combination of soundchips is rare.