Yeah. All at once would be a lot of posts flying around.
But one at a time would be a lot of waiting on everyone's part.
On the plus sides:
One at a time, as Scar said, means our characters get to watch. And the overseers might see that as important.
All at once: we'll get done and get to move on to other things sooner.
I'm personally of the opinion that all at once is better. Because the main advantage to one at a time is really only an advantage to the characters. Remember that, story aside, this is a game, and the point is to have fun. Waiting around and watching while others play just isn't as enjoyable.