It's not that easy, and has nothing to do with the spl. You have to reprogram the phone along with loading a proper PRL to get it to work on Sprint again. Reprogramming the phone involves going into the EPST settings and changing several different things. Flashing the sprint stock shipped rom might do some of that reprogramming for you, but i'm not sure. You can get the sprint stock shipped rom here
http://forum.ppcgeeks.com/showthread.php?t=42741.
You can't use both at the same time. I would suggest you just stick with one of them, cause it's not that easy to reprogram the phone and get it re-activated. You can switch back and forth between sprint and metro, but it's difficult work for a beginner.