Hope I'm posting in the right section. I think I know the answer to my own question, but I'm just after confirmation.

I'm running an MT-32 and an SC-55. The MIDI is chained from the 32 to the 55, with separate audio outputs running through a small Behringer mixer.

As they are sharing a USB MIDI interface, DOSBox is only able to send MIDI to everything. So both devices will play every soundtrack, and I control the one I hear via the channel controls on the Behringer mixer. If I was using other MIDI inputs such as an instrument or DAW, I would be able to use channels to control which MIDI synth was playing.

This is just how it is, yeah?

Curious to hear how others manage this situation.