Probably just run the HD7850.
there are people running HD7750 video cards dual graphics with 5600K on some motherboards,
HD7850 is different chipset, unlikely.
Dual Crossfire is difficult, originally intended with HD6670, 6570,6450, if I recall correctly.
The CPU iGPU is not fast, nor is motherboard memory. Anyway to match, if it can, faster cards have o slow down to share functionality.
There are web posts with 7750, I seem to remember 7770 as possible. The latest chipset is supposed to crossfire better. Faster motherboard memory scales best (which gets expensive.
There were some review posts attempting Dual Crossfire, generally a discrete video card is better and simpler.