Dual 6850s running on a single 1920x1080 screen should net decent results. However, for the most part this game isn't going to scale overly well with crossfire. The Hero engine just sucks at it, my time in SWTOR taught me that, running dual 6850s, then dual 7950s and now dual 290Xs and getting roughly the same performance out of the lot just less GPU useage (100%/100%, 65%/40% and crazy random percentages jumping back and forth between the two cards) I forced crossfire on ESO (using the SWTOR settings) with Radeon Pro and it was rather sad. The game, due to Hero, is only single threaded, so on my dual 290Xs I had NO upgrade in performance, in fact it dropped due to it switching between cards. I tried every style that AMD supports as well, in the end I am forced to take average 30fps on eyefinity, when my system should be able to run it so much faster. Basically one thread isn't going to feed full settings to dual GPUs, run AI, networking and all that fun stuff very well.
Possible good news is, Idea Fabrik (the guys that currently own the Hero engine rights) are working on a multi-threaded version of the engine. If games like Elder Scrolls Online and Star Wars The Old Republic get the upgrade when it's done is to be seen, but there is a glimmer of hope.
In the end, I would really recommend a higher end card instead of running crossfire with those two cards. A 280X is going to be consistently faster than two 6850s in crossfire, and you will be able to use Mantle.
Desktop: FX8350 @4.7Ghz, Gigabyte 990FXA-UD7 Motherboard, 16GB Corsair Vengeance @1.7Ghz, Dual Asus R9 290X DCU2 @ 1.1Ghz, Kingston HyperX & V300 128GB SSDs, 4TB HDD, CM Inferno Gaming Mouse, Logitech G510, 3x Acer 223W Monitors@5040x1060, Antec DF-85, Corsair H100i, Corsair HX1050 PSU.
Laptop: MSI GX70: AMD A10-5750 @3.5Ghz, HD8970M @1.1Ghz, 16GB Corsair Vengeance @1.6Ghz, 250GB Kingston HyperX SSD, 1TB SSHD, 750GB Seagate, Gamdias Demeter Gaming Mouse, CM SF-17 Cooler.