I've often run dual GM games with my local group, and really the reason we do things is while 1 person is very good at creating a story they suck at rules/paperwork, and the other person may be a dictionary but not super creative. It's kind of like the artist/carpenter combo. One person creates the story, the other brings it to life. Also it helps take care of the differing ideas for the campaign.
Also, if you are interested in running such a campaign, I make an excellent paperwork sub-DM. I'm very good at creatively running encounters on the fly and predicting players (once I ran the same fight twice from two different perspectives, and I knew how my players would react so the first encounter was right :P)