[Bartholow]

Good call bob. Even though it doesn't look like you make money by betting, it is much better than check calling. Especially since people will often not raise the river when they make two pair.

Of course, against players who will bluff or even value bet aces on the river if you check, now you come out ahead with the check call. I think there is some great discussion of all this in Ray's book.

The default should be to still bet your two pair. Reasons not yet mentioned include not making it obvious when you have better than two pair, and making it more possible to bluff.