Bouncing Midi tracks to individual audio tracks

You can bounce synth audio output tracks to other tracks on a one-to-one basis, but if you've only set up one audio output track for a multi-timbral synth, you would have to solo one MIDI track at a time and bounce the audio for each part one at a time.
 
If you have a separate audio track for each output of the synth, you can bounce with Source = Tracks, and you'll get one track for each output, but usually you would just 'Freeze' the synth to generate an audio file for each ouput track 'in-place' without creating new tracks.
 
Also, note that most multitimbral synths provide a stereo audio output for each MIDI part/channel, but TTS-1 provides only 4 stereo outputs per instance that have to be shared if you use more that 4 parts/channels in one instance.
 
 

I depends on the synth you use and its routing.  Give us more info on your project and you'll get the answer.