As Long as they get there own space they should be fine. My big royal has decided to move in next to my Cactus and they are now always butting heads through this tiny gap betwwen their caves. I think it is more the Royal than the Cactus though. If you are getting 2 I would make sure you create two seperate territories, perhaps at each end, as if they decided to fight they can do alot of damage. This can happen during feeding time, so watch to make sure they are both actually getting enough food.
Like Henward said they are carnivorous and should be fed accordingly, with the odd algae wafer.
Overall a fantastic pleco, great to look at and if your lucky (like Henward) it will be an active one during feeding.