On having a co-gm:
A Voice in the Dark from "Components of a Successful Sandbox" msg #10:
Co-GM's are a bit of a sticky wicket. When using a Co-GM you should do so from the beginning. I've seen it done well, and I've seen it done terribly. Both need to either be already telling the same story, or one needs to be only dealing with one aspect of the game.
I'm currently a Co-GM in a game for a friend of mine, and my sole areas of responsibility is editing the posts. with a small amount of help regarding rules, though I don't make the rulings. I simply edit his and the player's posts for readability and grammar. (My friend has speech issues and asked for that help.)
I've seen games with Co-GM's that were able to work well, and other's that died from GM infighting/confusion. The ones that worked well were those that followed the rules above. I believe that the key to a success in that situation is the same as in most. Communication.
On DnD on RPoL...
Following the initiative order like normal (in series) may slow things down, because for each person's turn there will be a delay as you wait for them to post. In person it's not so bad, but on play by post it could add up.
One solution might be: when a new round begins, just tell everyone to post their actions, and then once everyone has posted, things play out per initiative order.
Players can do a little "if this happens, do X, if that happens, do Y." You might just have to accept that some flexibility will be lost for the sake of keeping things going. Or, if something really big and unexpected happens partway through the initiative order, you allow those remaining to adjust their actions.
Another solution would be to have phases. Team A phase, then team B phase. Or break it up a little, team A1 phase (the faster people on team A), then team B1 phase, then team A2, then team B2.
This might reduce the weight of good initiative, or lead to power combos, but it's an idea.