Well, for mutual survival. The idea was that the races momentarily put aside their differences so that they could gather enough great mages to open the portals in the first place. 

Howeveeeeeeeeeeeeeeeer,

If it is really incompatible, then you could mention that perhaps your race stormed one of the complexes being used to create the portals, slaughtered or drove off those who were waiting to walk through, and hijacked it?