Are you talking about mixing the outputs? I have both output into a Behringer RX1602 (along with a couple of other vintage PCs and my MIDI sound modules). Much better than connecting output of one to line-in of the other.
Or are you talking about having both cards present in the same system? I'm not as well-endowed as Shponglefan when it comes to GUS cards, with just two - but both are running next to SB-compatible cards, in my most-system (a P3-500 late DOS box) it's a GUS Classic next to an AWE64 Gold and an Aztech 4th gen SBPro2 clone with OPL3 and hardware MPU-401.
My resources:
Aztech card:
A220 I5 D1 P330 T4
AWE64:
A240 I7 D3 H5 P300 E620 T6
GUS:
A260 I3 (both) D6 (both)
Works fine; for AdLib both Aztech and AWE output, depending on which one I listen to I get OPL or CQM.