I do not see why your XIGMATEK MC NRP-MC851 850W would not power 2 4870X2 in CF. On AMD's website of recommended power supplies, they list the Xscale EP-750P3A as one of the recommended power supplies.
It has +12v1 20a, +12v2 20a, +12v3 20a, +12v4 20a. With a total wattage of 750w.
Your PSU is very comparable to that one, and it's on the recommended list.
I don't see any reason why not, but maybe TheGreatGrapeApe or someone else can provide some more insight.