No they will not SLI in any way, you could always try using the 550 for physics but it is more or less a waste, you can pick up another 460 used for an extra 20 bucks if you sold the 550ti.
Well, the 550 would be coming from my brother which is upgrading to a 660, and I will be getting a 660 too as soon as I can. So I was just wondering if I could get some better fps until then.