EDIT: What I mean is, can I mix the stereo track internally (inside the MPC) with different tracks on it?[/quote]
Yes you can, sort of. They are assignable outs Like if you wanted to separate your kick to out 1, snare to out 2, some hats and shit to 3 & 4, then you can still have a stereo mix of whatever other tracks you want out of L&R together. Obviously you can assign your tracks, so it you could use 1&2 for a stereo mix of a sample or something, and L & R separately for your other stuff... But if you put your kick to output 1, it's not going to come out of output L any more - as an example.
And keep in mind that mono samples usually let you scroll through (1, 2, 3, 4, L, R) while you assign them. Stereo tracks will likely require groups (12, 34, LR).
"Rap is an art, you can't own no loops".
-Guru