add the total number of each tribes powergraph rating together. Example, in a 2v2, one tribe is 1st and 4th, one is 2nd and 3rd. 1+4=5 and 2+3=5, this is the only tie, then when go to combined total civilization score. Basically, 1st and 2nd, or 1st and 3rd will win.
hey, it's an idea...