SLI at your own risk, micro-stutering happens alot... not everyone cares about it
but some regreat buying a 2nd card because they feel they have better performance with just one, micro-stuttering happens even at high FPS.
You can play Ultra with 560ti SLI 1GB, you just have to make sure you have 8GB of system memory for it to work without any stutter, I'm saying this from experience , not guessing, look at my system specs. It will stutter with 4GB or less system ram.
Edit : of course the 560ti 2GB is a better choice, but the 560ti 1GB is possible.