Mixing them will dull the recreational effects from each, since it dirties up the highs by mixing them together, but the mix will increase the overall euphoria and focus than taking them separately, as Dopamine is Dopamine to the brain.
I've never used amps recreationally, but I would guess the best would be to take some adderall, then kill the comedown with the oxy, allowing you to feel both drugs and saving something for the comedown.
I have mixed adderall and hydrocodone a couple times, couldn't really feel the hydro, I was too Geeked... But I agree with Ergic, if I was you, I would save the oxy for the comedown, which was, for me, always HELL.
But, if you MUST take both simultaneously, I would suggest taking a little less of both drugs, to test the waters, so to speak. Goodluck!