There have been some reports today that Samsung is producing 40% of the A8 dies. (Apparently sourced from IHS iSuppli to Re/code). I've seen no photographic evidence of this, whereas I have seen evidence (from ChipWorks) that A8 is manufactured by TSMC.
I doubt this Samsung "news."
First, it's not clear to me that IHS iSuppli even has the technical capability to make this determination, so they may simply be reacting to whispers from the magical "supply chain sources."
Second, TSMC and Samsung have different "design rules." "Design Rules" refers to the set of rules that chip designers must follow. For example, a design rule may specify that the minimum surface area of any piece of metal must be "X" or the spacing between two pieces of metal must be "Y" on metal layer 3. Different fabs also use different processes, meaning wire heights are different, transistors have different properties, etc.
It would be extremely difficult to design for two different fabs, each using a different process, simultaneously. The design effort would be around 90% of the design effort of designing two completely different chips. The only way to do it practically would be for Apple to create their own "worst case" design rules - a sort of "least common denominator." This would leave a lot of performance and power savings on the table. This would still entail difficulties as path timings will differ between the fabs, and fixing a critical path on Fab A's chip could cause a hold time violation on Fab B's chip.
Anything's possible, but this seems unlikely.
If it's true, it goes a long way toward explaining why A8 didn't make a much bigger leap in performance over A7.