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Enemy of my Enemy is my friend.

As long as both sides benefit from the alliance (And thus, the inevitable betrayal), there is no reason at all for any two factions to join forces.

If you brutally deflect a Tyranid Hive fleet to a Tau planet, You may not be working 'with' Tyranids, but you certainly benefit from either out come of the war. I believe the Eldar (?) did this exact scenario once. but it may have been IoM, not sure.