I've been looking at big wall harnesses and I've been noticing that a lot of them come with two belay loops:
I know that it isn't there as a back up. What's the advantage of having two belay loops on your harness when aid climbing?
It's simply for organization sake.
Aid climbing on big-walls involves lots of gear, and you'll very quickly realize how much of a mess your perfectly racked gear will become.
With two belay loops you can separate your aid ladders and daisy chains a little, which helps to keep them from wrapping around each other (and keep things a little bit cleaner too).
Couldn't find a good close-up picture for you, but Pete's picture from MP is a good visual indication of exactly how much is going on around your belay loop sometimes when climbing big-walls.
Chris McNamara talks about it a more in his book on big wall climbing also. http://www.supertopo.com/a/How-To-Big-Wall-Climb-Table-of-Contents/a139n.html