You need to SLI identical cards. So it wouldn't work.
Also if I may ask, do you have performance problems right now in your games? If not, I don't see the point. I'm not a big fan of SLI to start with, I'd rather have just one good single card instead. SLI increases the complexity of the system, requires a beefier PSU, consumes more electricity, generates more heat in your case, more chances of running into drivers problems, etc...
A GTX-660-Ti was a card released last year, so it's still pretty good. As I said, if all your games run great, I don't see the point of modifying anything.