I think this is a fundamental misunderstanding (or lack of a feature, depending on how you look at it).
PD can't "assign and combine" tracks. It can only play them all at the same time. Another way of looking at it is that you can't assign a track to a particular output channel.
On the video side, if you want to make a 3D video you need to start with a 3D video. You can't take two different video tracks and say "#1 is the left eye, #2 is the right eye."
On the audio side, you can't assign different audio tracks to different speakers.
That sure would be a nice feature.
Jerry Schwartz