Why won’t the motherboard work with two CPUs

I (very) recently built a computer with the following specifications:

• (x2) Intel Xeon X5650;
• Supermicro X8DTI-F Motherboard;
• AMD Radeon R9 280X;
• 16GB DDR3 FB 1333MHz RAM (4x 4GB);
• 1TB Seagate Constellation SED (enterprise HDD);
• RM850 PSU

Now, the system runs perfectly fine with one CPU installed (installed Windows, games, programming, etc, are fine), but the system will not initialize both CPUs when both are installed.

I have a PCI POST card (the same as shown below), and it stays on '00' when both CPUs are installed; the same code it uses when no CPUs are installed:

Both CPUs work and so do both sockets (basically, everything works except when I try to use both CPUs at once), so I'm not entirely sure what's going on here.
I have also tried another PSU and resetting the CMOS, none have worked.

There is a BIOS update available for my motherboard, but applying it hasn't fixed the issue either.

Here's the details in CPU-Z of each CPU:

I took a screenshot of CPU-Z, and then shut-down and replaced CPU #1 with CPU #2.

To recap:

• Not a power issue (I'm using the correct cables);
• Not a CPU issue (others have been able to get the exact same CPUs to work with the same motherboard);
• No issue with the actual sockets (no bent pins);
• Updated BIOS, no change
• Reset CMOS, no change

What could be wrong here?

The seller of the motherboard is sending me a replacement board that has been tested to work with dual CPUs, but I'm wondering if there could be something simple that I'm missing?

I've looked into it in more detail and the QPI is not going to its full 6.4GHz (even when forced); instead it's throttling to 3.2GHz, sacrificing some of the Xeon's performance.