I don't think I've ever seen a LL specify any preference for one set or two. As ponygirl says, there's often a rule that pics should be in sequence - usually that the model starts off clothed, say, and ends up naked (saw a set the other day where the girl starts off naked then gets dressed in some latex gear - was tempted to submit it just see the reaction

). Also some lists (can't recall which offhand) specify that you can't use a random collection of themed pics in a gallery - say 10 pantyhose pics featuring different models.
I usually use a different set in each gallery, but I'd be surprised if there was any problem with using one set over two galleries provided they're arranged in a logical sequence.
Rob.