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.