They could be competitive, in which case the aggressive yeast will stand in the end and the other yeast will be eradicated.
The best way to mix yeasts is to add the yeast from which you want to derive primary flavor characteristics (generally esters) up front, and add the second yeast at or immediately post peak fermentation for the first yeast. This works (for example) with Windsor up front and Nottingham following behind, wherein the flavor esters from the Windsor are departed into the forming beer, and then the comparatively esterless Nottingham (added second) eventually kicks in and takes the FG down well below where Windsor would have taken it, but the Nottingham can not take away the already produced Windsor esters. Windsor can reach peak fermentation within 24-36 hours, so once you see it start bubbling, add Nottingham 24 to no more than 36 hours later.
I once pitched Windsor and S-04 together right up front, and I'm somewhat convinced that the S-04 quickly wiped out the Windsor.
