Hmm, I'm not 100% sure what you're asking, Merilin.
I have heard that with the MMR, 95% of kids vaccinated become immune. They do a booster shot a few years later to get some of the kids who didn't become immune with the first shot. (95% chance of immunity is good enough for me.) They don't usually do a third dose of MMR, around here at least.
But that doesn't have anything to do with splitting up the MMR. When we talk about splitting it up, we mean splitting it into its component parts - one shot for measles, one for mumps, and one for rubella. That way we can space out the shots so there isn't exposure to all three viruses at the same time, and some of us will just do one or two of the components - just the measles, etc. I would assume that the first dose of the measles-only vaccine would give a similar chance at immunity, about 95% (but I haven't found statistics anywhere to be sure). So, if you got the measles-only shot, a doctor would still tell you that you needed a second dose to maximize your chances of becoming immune.
Did that clarify anything? Or did I miss the point completely?