in the mean time......
Once you get one track done..... simply do a BOUNCE TO TRACK on the synth. MC5 will take it's time and render the synth to a new track as an audio file....(WAVE FILE) which should play just fine.
now.... hopefully you have added the tracks manually and if so, "disconnecting" them from each other is easy.
After you disconnect the midi source track from the synth, drag it to the bottom and let it set there. A disconnected midi track uses NO CPU power in the project. Archive it or hide it if you wish. It can be unhidden later if you need it.
Now.... simply insert a new midi track..... connect the new midi track to the synth and change the patch or the samples and create the next track in staff/PRV.
Repeat the process by bouncing, then do it all again.... one track at a time. I have heard a few people's work here that was amazing and they used this very method.
MC5 and a factory card can often run the audio tracks pretty well. It's the real time midi rendering that causes issues for factory cards. Getting a nice after market interface will solve that issue and let you run multiple synths in real time with no clicks and pops....depending of course on the computer's ability and specs.
Hope this helps you some.