Can the motherboard X9DRW-3F with 2 CPU install only 2 pieces of 8GB memory, total 16GB? On your manual, it seems you need at least 2 DIMMs per CPU. If possible, is it installed at P1-DIMMA1 & P2-DIMME1?
There's no problem to use2 DIMMs with 2 CPUs. And yes, in the slots you mentioned.