Yea, it sucks. I don't have to deal with split face, but trying to get pictures of a white dog, a black dog, and a white and black dog all together is pretty much the same level of suck.
My only advice beyond what's already been said (and I agree it's easier to lighten black than dampen whited-out white) is that direct sunlight is a killer (better for the black but just absolutely whites out the white) and avoiding high contrast backgrounds if you can will help a lot.