if you run Seti at all times (meaning, not in just screen saver, but rather all the time), Seti will take precedence over RC5. But like DanC said, many people run RC5 (less overhead on system resources) while working, and let S@H kick in for the screen saver.
So you can run both at the same time, but really, if both are running continuosly, S@H will use up all your cycles.