At some of the larger restaurants like 'Ohana's, Chef Mickey's, and even surprisingly (to me) Chefs de France and Big River Grill on the Boardwalk, our parties of 10-15 have been seated together. However, at some of the ones that are more cramped for space (Liberty Tree Tavern, Le Cellier, etc), we've had to sit at different tables.
For the most part, they try to sit the tables close to one another, but there have been times when we've had a few different people between us. Unfortunately, it's one of the lumps you have to take with going down together as a big group.
That being said, at least you're going into it realizing you may be split...when you're not, it'll be that much more fun!