It's customary, from what I've seen, to have an "out of character" thread for bookkeeping type things, like if someone's going to be AFK for an extended period or something, and an "in character" thread for the game itself. I'm not sure if it's necessary though. I think everything could be discussed in one thread with some method for participants to specify when they are speaking as their character, so it depends on how separate you want to keep things, which might reduce clutter. The OOC thread, or even a third thread, is sometimes also used for recruitment, although it sounds like you already have your group so probably wouldn't need that. I've also seen a separate "rogue's gallery" thread where the PC sheets are posted and updated. I've used the recruitment thread for this before.