Sometimes mixing works and you get the best of both worlds. Sometime not so much. I would read up on both yeasts and try to give one a head start if you wanted it to dominate. So if one starts faster than the other, pitch both at the same time in the same condition. If they take off about the same, try pitching one at high kroysen, and the other from a dormant state. The high kroysen should take off faster... you could also try pitching one, and then pitching the other 12 hours later.
While I agree with Denny that the results may be somewhat unpredictable, the few times I have tried it, the results were fairly similar. I suspect from a spectrometer standpoint, the results are different, but from a flavor threshold standpoint, they came out very similar. (I did not like the belgian, american ale yeast results..but they were repeatable).
Good luck, try it, you may find a magic mixture!