Precision 7920 Mixed CPU support

Hello! I'm having a hard time finding any info on this. Nothing in any manual I've been able to find and no threads here or elsewhere. There was one thread that looked promising but the answer was about PSU and not CPU. Whoops!

So here's the question, I'm looking to upgrade the computing power on my 7920 and I'm not sure whether the best plan would be to replace the current CPU with something beefier, or to purchase a 2nd one and populate the 2nd socket.

The main question I have is, do these systems support using 2 different CPU models in each socket? As stated I cannot find any documentation saying that they will or won't. Anybody had a chance to try this?

Cheers!