ASUS Z8PS-D12-1U does not recognize two (2) CPU's

[JonSmythe]

Hi!

ASUS Z8PS-D12-1U does not recognize two (2) CPU's.

Putting a CPU into socket-1 only: boots

Putting a CPU into socket-2 only: boots

Putting CPU's into BOTH socket-1 AND socket-2: does NOT boot

Is there a BIOS setting we're missing?

Something else?

Any help is much appreciated!