Using Digital Performer & Mainstage together LIVE

It depends on your latency requirements. If you are not super sensitive to latency, then you can probably achieve what you want using either soundflower (from cycling 74, search google -- it will let you route audio from mainstage to dp, or the other way around), or by using two different computers and physically routing the signal. The second option would probably be more preferable, since in a live situation DP and MainStage will probably compete for resources on your machine... not sure what kind of effects that might have on audio dropouts or other audio problems.

i don't think soundflower is necessary.
Just simply use difference audio devices for each (built-in for one, third-party i/o for the other).

Doesn't seem like a good idea, though. Both are real-time apps demanding a lot of the system.