Hmmss ... no answer ... but here is my solution :

When an ally of mine is attacking the same province as my army (doesn´t happen often but had quite a few of those), I usualy lead my men to a nice high ground and enjoy the show of both AI´s tearing each other apart.

When the "show" is over, there are two kind of scenarios.

1st. Your ally wins. If so either you or your ally gets the province ... duno of any reasons, like whom get´s it.

2nd. The Enemy routes your ally from the Battlefield ... and thats the way (aha, aha), I like it ! Use you Cavelery to ride down some of your enemies elite units as they chase your ally, but spare some so that they chase your ally all the way back home.

With your ally running, it is now your turn alone to take the field. You can easily use the disorder of your enemy and crush him anywhich way you like, and with you beeing the only remnand of your `alliance´ ... "And the Province goes to Player 1 !!" *applause*