Well, I got it working. Digital and AdLib sounds are coming from my CT2800, but the MT-32 is being driven by my CT2740's MIDI port.
I chose my CT2740 because it has the MIDI port set by a jumper, rather than through software.
One thing that I sort of had to guess was using the DIAGNOSE /S command appropriately.
My AUTOEXEC.BAT has the following lines in it:
SET BLASTER=A220 I5 D1 H5 T6 P300
SET BLASTER=A220 I5 D1 H5 T6 P330
The reasoning behind this is that it "activates" the Vibra 16S card with the MIDI port set to 300 as per BLASTER, but then changes the BLASTER environment back to using P330 so games will detect it and send their GM stuff thru the SB16's MIDI system. I originally tried a CT2940 as the secondary card, but it's a PnP card and CTCU wouldn't let me change anything.
Does that sound right? It seems to work OK so far...
Guess I'll dig up Hexen and try the warp 2 test.