Well, I can tell you this: I'm taking a 1/4 of generic Proscar per day, and it's definitely working, so if splitting the pill "kills the drug" then I'm having one hell of a placebo effect!
Do a search on cutting proscar. I think there was something on the proper way of storing it as well. I would use a pill cutter and cut it into 4ths. Store it in a old pill bottle or when of those small pill boxes you can buy at a department store.