You can't 'SLI' them - IE to work together for increased performance, but you can run two cards at the same time though and have a monitor/display attached to each card without any issues (unless drivers only support one and not the other, which is possible with cards that far apart in age).
If you use 2 different cards then both will work but there simply won't be an option to enable SLI in the driver's panel.
Nvidia locked any way to SLI different cards in new drivers so you need the same GPU to make it work. Modded drivers etc were sometimes working in the past but I'm not sure if it's still working. At least many users in the linked thread can't make it work on new hardware.
AMD let you set Crossfire on 2 different cards from the same family but it's also not working with all cards ( there was a table on their website ).
In this case I just see no point to SLI 8600 with gtx660. I mean 8600 will only slow down everything even if it will work in sli.