So Wanting to run in SLI mode without flickering here is what I did:

First make sure you are using the latest BETA drivers & you have the latest SLI profile from Nvidia:

http://www.nvidia.com/Download/Find.aspx?lang=en-us
http://www.nvidia.com/object/nvidia-multigpu-SLI-profile-update-v7-driver.html
(it says it's for the demo, but works for the game too)

Add the following to both of your cfg files (autoexec & system):

r_MultiGPU = 1

Make sure the following line is commented out or remove from autoexec.cfg:

;sys_spec_postprocessing=3

Explanations and exact cfg files to follow

Try it out... I hope it works for you.

-------------------------------

edit (3/28/11): It looks like those who have gotten it to work with Crossfire are using the 11.4 Preview drivers & it only works in Single Player, hope that helps out the Crossfire users too.
Only way to fix it for XFX is to run the game with bioshock profile from RadeonPRO however you do lose performance. The only other option is to use the 1.0 .exe
-------------------------------

edit (3/27/11):

Wasdie has updated his config maker to include extreme settings & the MultiGPU fix, however it still leaves in the Post Processing line so you will need to go in and remove it. His app can be found here:

http://www.gamesas.com/forums/viewtopic.php?f=40&t=12953

Also the mouse Acceleration & Smoothing setting in the app are a little buggy so you will want to double check that those settings were applied correctly. I also updated the config files below to reflect the changes.

-------------------------------

P.S. @Crysis Team / EA -- I am currently an unemployed test engineer... I live about 5 miles away from you Salt Lake EA office if you want to hire me. =)

It looks like there is a glitch in the processing of SLI where you graphics cards are trying to run SLI mode & Crysis 2 is trying to run SLI mode and for some reason they are conflicting...

By default Crysis 2 has this line (r_MultiGPU = 2) which is SLI/XF Autodetect, but for some reason that is busted. (0 if off & 1 is on) (thanks nuclearnav for the info, you are correct)

if you set:

r_MultiGPU = 1

then SLI mode works fine, if you have GPU monitors you will see that both GPUs are being used, there are also some other tweeks I am using to minimize the stress on my graphics cards.

You need to place the r_MultiGPU = 1 in both the system.cfg files & the autoexec.cfg file for it to work 100% of the time, if you leave it out of the system.cfg the multiplayer doesn't work; if you leave it out of autoexec.cfg then it only works about 1/2 the time. I don't know why this is, but it's what I have found.

Here is my autoexec.cfg file:

---

---

---

---

---

---

---

---

---

---

---

---

---

---

---

---