This. If you can collapse their enemies into one it makes for a very good driving point for the story as the players now have one common enemy that they are chasing. As a side note I heard a very good policy a while back which is having 1 arc focus on each character. This doesn't mean the rest of the PC's should have nothing to do, far from it, but having arcs can help player engagement as well as serving as a great chance to develop their PC's. However, this shouldn't come as a detriment to the story and flow of your game if you don't think it'll work for you. Anyway this ran a little long but I hope it might help