If a new mutation happens, I think it's almost sure to only happen on one side of the paring and not both at the same time. This is to say that almost surely the first het albino was hatched before the first albino. The mutation happened once to produce the first het albino which passed on the mutation to some of it's offspring and eventually some time later two het albinos bred together to produce the first homozygous albino. The chance of two homozygous normal for albinos both having a spontaneous albino mutation in the same sperm and egg that are destined to meet and produce an albino are astronomical. If a spontaneous mutation is a 1 in 100,000 chance then the chance of it happening on both sides is a 1 in 100 billion chance.

Now the albino mutation might have happened more than once over the history of ball pythons and more than one line may have survived. Perhaps all the het albinos aren't descendent from the same original het albino. And, once you have one side of your captive pairing down as known to have the albino mutation then it's possible a spontaneous albino mutation could happen on the other side to produce an albino. But the times (and there have been several) that breeders have unexpectedly produced albinos from breeding a cb albino carrying male to a presumed normal female it's far more likely that the female was an unknown het all along descendent from an already existing line carrying the albino gene and no one knew it.