Woo Hoo!
That excellent website
http://www.baseball-reference.com/
now has splits—including platoon splits—for modern players. (E.g. Mickey and Reggie, but not the Babe).
Of course, you can also get splits from
http://www.retrosheet.org/
but the baseball-reference splits are easier to access for reasons that should be obvious to anyone who has gone through retrosheet's odd, multi-layered alphabet system. The splits pages at baseball-ref also seem to me to be easier to read.
I just bumped into this new feature at baseball-ref by chance and thought you might want to know. It's part of a what looks like a major upgrade of the stats and info available at the site.