You can get a Sound Canvas 55 relatively cheap if you're patient. They often come on the market for €50 or less at which price they're well worth the price. The MT-32 is less easy to get. I was patient and managed to buy it for €90 (and my SC55 for €40) and since then I bought a spare SC55 AND also bought a RA-50 which is very similar to the MT-32 and is quite a bit cheaper most of the time.
For a setup I recommend you go to Serdashop and get the DB15MIDI. This will let you output two MIDI signals. You can buy midi cables with two cables into one (for two-way communication I take it) which are perfect for this: you split them off at the end and feed one into the SC-55 and one into the MT-32 (or RA-50). If a game uses MT-32, just mute or turn off the SC-55 and visa versa.
Personally, if you're serious about your games and retro gaming, I'd just save up for them. The experience is just far more authentic - seeing the screen light up with "Insert buckazoid" and being able to quickly change volume right in front of you (I have the two underneath the monitor with a wooden laptop holder - works great) is just so easy.
Also, and this is a big one: IF YOU WANT THE GAME TO SOUND AS THE COMPOSER INTENDED, SC55 AND MT-32 ARE PRETTY MUCH THE ONLY WAY TO DO THIS FOR A GREAT MANY GAMES. Many many games first had their music made this way and then ported to Adlib etc. which is why so many games had Roland support in the first place despite so few people owning Roland devices.
Yesterday I received my Orpheus - this will also negate the use for SOFTMPU which is not compatible with all MT-32 games. I'll sure have a blast testing all this ...
Retro game fanatic.
IBM PS1 386SX25 - 4MB
IBM Aptiva 486SX33 - 8MB - 2GB CF - SB16
IBM PC350 P233MMX - 64MB - 32GB SSD - AWE64 - Voodoo2
PIII600 - 320MB - 480GB SSD - SB Live! - GF4 Ti 4200
i5-2500k - 3GB - SB Audigy 2 - HD 4870