Well, I dunno too much about tag mode since i rarely find myself playing it. But when I do, I tend to use Sub Zero or Kitana as my partners.
I think Kitana gives that rushdown pressure Mileena can lack at times, while Mileena in turn provides counter-zoning to aid Kitana. As for Sub, well he slows the pace of the game down for the opponent so he's almost opposite of Mileena. This can be good too, since you have to completely different fighting styles for the opponent to try and adapt to.