Something that I've seen work really well before (outside of forum rps, mind you) is have one GM handle to "day to day" things and side-stories. Buying stuff in shops, smaller encounters, and so on. Meanwhile, the main one runs the main story and handles the big set pieces where you get really involved battles or whathaveyou.

Of course, you need someone you can trust that can handle all the minor stuff, but if the two of you can work together it can turn out to be a real banger.

One tip I have for doing it is work very closely with the co-GM, giving them knowledge on upcoming story bits and things to sprinkle into their minor plots to help tie everything back into the main story.

Hope this helped!