I ran gtx670 and radeon 5970 at the same time. Playing on nvidia and mining on radeon.
Nvidia was on hdmi and radeon was on dvi. Both plugged into the same monitor at once.
I had to change my input to dvi, then go to the catalyst control panel and set the radeon to that desktop (in my case it was #2).
Then go back to hdmi input and set nvidia to the other desktop (#1).

After that I was able to change speeds in msi afterburner and mine.