Having two GMs run seperate parties in the same game world can work, but there are issues. Chronology can be an issue you will have with two GMs.
Chronology is also an issue when a party splits up and one person posts more than another. Or when a wizard takes 100 days out to build a magic item. Oops in the chronology, by taking 100 days out, you missed your opportunity to drop the Ring of Power in Mount Doom. Sauron has won. Game over.
It might also be easier to start one new group of PCs and have that group encounter existing groups in other threads. That solves the multiple GM problem.
However, when you go join an existing thread, you can run into house rules issues. For instance, the "House Millithor in the City of the Spider Queen" thread that I GM would seem ideal for a visting party of PCs (levels are around 12th, 3.5 rules, etc.), but even I have some minimal house rules (i.e. I don't allow PRCs).
Good Luck.
(I can't join any more games, not time.)