I have never been lucky enough to have both (sli and dual screen set-up) - I think you can't also, but just don't know.
I think thats the stroy for most here I am afraid Zax mate - people are either SLI or dual, not both.
I believe you can't because thats one fo the reasons why Nvidia upgraded the drivers so SLI can be turned on and off in windows (so when u r not gaming dual can be switched on, when u r gaming SLI can be enabled).
Give it a got and report back.
Cheers
MAv