Splits should be dealt with immediately, Lizzie! I would see which branch involved was smaller, and cut it off. Then I would use a chisel and a plane or smoother (looks like a cheese grater with attitude) to eliminate the split and smooth the wound. I would also seal the wound immediately afterwards. If the tree winds up seriously lopsided, I would also lightly prune what's left to balance it a bit. If it's split into the main trunk, the only thing to do is cut the tree down and seal the stump--it will regrow over the next 2-4 years.