That's pretty messed up that the were either lying or simply didn't have their staff on the same page. As for your dilemma, check in on both booths during preview night/Thursday to see what requirements there are (tickets, random drawing, just show up, etc). If one or both give you tickets/wristbands there is a chance for you to get both. Go to one of the signings super early (or as early as they will allow you) and be one of the first to get a signing, then when done, head on over and go to the next line and be at the end. It may involve a big block of time, but you can do.
Otherwise, pick which one you really want and stick with it.
And like others asked, get confirmation, then ask for confirmation from someone else just to make sure.