I dont think its all that uncommon. I hear about them every once in a awhile. They happen from pairing multiple males with them same female. If you put a spider and pinstripe male in with a normal female you could end up hatching out spiders AND pins. You cant hatch out spinners though. By split clutch you mean a multi sired clutch right?
I'm not sure of the odds but it happens quite often. Some times it's split down the middle but in my experiend it's more often that you get one or two hatchlings that have a different father. Here is a clutch from a few years ago that had at least 3 fathers. I bred pied, lesser, pewter and bumble bee males to a pied female.