When I load multiple instruments into something like Kontakt, it automatically assigns the next midi channel number in order, as each instrument is loaded, so the 1st instrument is assigned to midi channel 1, the 2nd gets midi channel 2, etc.
By default, however, all of the audio will still route to the same main outputs, even if I am using a version of Kontakt that has 64 outputs available to it.
I have to change the audio routing of each loaded instrument to tell Kontakt to use different audio output channels for each instrument, if that is what I want, which always is the case.
I then insert a pair of mono audio tracks and a single midi track into Sonar, for each of the loaded instruments, then for each midi track I route the output to Kontakt, but set that channel strip Midi Output Channel (right under the FX bin of the midi track), to the channel that corresponds to one of the loaded instruments in Kontakt. I also route a pair of audio tracks to the stereo output of the corresponding loaded instrument from Kontakt, and repeat this process for the routing assignment for all of the loaded instruments in Kontakt.
THEN, using the Sonar default of Always Echo the Current Track for midi tracks, when I switch focus from one midi track to another, Sonar will turn OFF midi input echo for the track going OUT of focus, and Sonar will then automatically turn ON midi input echo for the midi track coming INTO focus.
When all of the above is set up, I will hear ONLY the instrument for the midi track that is in focus.
I would think the same sort of thing would work for your soft synth too, or would be quite similar.
Bob Bone