Spot on Martok.

It is definately cheesy but in our defence I have lost count of the amount of times that that an ally just happens to chooe to invade the same time you do.
At one stage I sussed this out by quick saving the scenario and putting a full stack into the province, and my ally was sending in about 100 more troops.
Well I reloaded the game and only send in 1/2 a stack and guess what, well my ally did the same with about 50 more troops than I.
About 4 different scenarios and every time the ally troops numbers changed but always so they had just a few more men so it is no coincidence the timing of the ally invasion, or the underlying intention behind it.