I doubt Apple would have two suppliers for the same chip. The two fabs use completely different design rules, so they'd have to have two separate designs. Not impossible, but doesn't fit with how Apple has done things before, unless they're doing it to allow squeezing both on price.
Seems more likely that there are multiple varieties of A8 (i.e. like the A6/A6X) and one would be made by one fab and the other by the other fab.