That wil not be possible, you can only SLI matched cards, you can SLI difderent brands, different clocks and even vRAM size, but no different models, and 650 are 3.. 650, 650TI and 650TI boost. And yes as far i know as said above only GTX 650 TI boost can SLI..
The 650Ti boost are the only ones that can be ran in SLI. the other 650s don't have the connectors for SLI bridges. Can can have 2 of them in your machine, but not using SLI, kinda crazy really. Nvidia should have at least made the 650Ti be SLI ready.
I wish I could SLI my 650Ti in my htpc, I'm curious to see what kind of performance two 650Ti's would have. I could always sale my 650Ti and upgrade to a boost, but that just seems like a lot of trouble.