mmmm, do you mean 460 GTX SLI + 570GTX so I can plug more than 2 monitors?
I can't answer for sure about adding the 570GTX to the mix, but I do know you cannot tri-sli 460GTXs.
Are these 768MB 460's? If so you would probably be better off running the 570 by itself for gaming. But if you are wanting to run 3 or more monitors I guess the 460GTX SLI would be your best bet.