I guess you could if you have Lucidlogix virtu or something similar. you can run them both separately, but you might run them into driver issues running them together....I would just run the gtx 460 alone
you can, in fact, run both nvidia and ati cards together on the same computer under windows 7 without issues, and even put them in hybrid crossfire/sli and have the nvidia gpu for physx.
Ok, no problems so far.
and you can have both cards running independently and each one feeding different displays, which could be beneficial since it would avoid the issue of having the primary display running at 3D clock speeds all the time.