Our DD dressed in her costume before we got there. It was just easier that way.
As far as posing two together........of course you can!!! What I'd do, is have one go first, then have the two of them together, and have the remaining princess finish up. You'll have to specify to the photographer what you want though. They are very accomodating. We did the photo shoot for our DD, then we joined her. It was great!!!
Can't wait until Monday the 19th............she and my neice are doing it again!!!