Multiple Graphics Cards to Render

Is there a way to use multiple graphics cards at once to render? I have both GTS 250 and GTX 660 Ti but Blender only seems to give the option to choose between them rather than utilise both.
Cheers

I am pretty sure there should be an option to enable all of them in the preferences… which version of blender are you running?

V2.67
http://i.snag.gy/EgVyt.jpg