In the BBC version Caspian is very young. In the book Caspian is about the same age as Peter, but not as mature or tried. I don't know anything about how this movie is being played out. But to try to make rivals out of Peter and Caspian would be a waste of time in character development. The book is already full of character development to bring in more. There are so many characters that each need to be brought out in the movie. In LWW there was just the 4 children, Aslan, the witch, the beavers, and Mr Taumus. In PC, you have the 4 children, which are all very much developed in new roles, Aslan, Caspian, the two dwarfs (both very unique), the badger, the squirrel, Reepicheep, Miraz, Dr. Cornelius, plus minor characters like the bears, the giant, the moles, Trees, Bacchus and Silenus, and Sopespian and Glozelle. PC is a story full of characters that must be given time in order to complete the story. Adding other story points means you may have to cut out other character developent. You already have a lot of rivalry in Caspian's council.