Gtx 560 ti 1 gb and 2 gb sli

I believe you can. My understanding is that it will read the entire setup as 1gb of memory due to the smaller card however so you may actually lose some performance over having the single 2gb 560ti.

http://www.geforce.com/hardware/technology/sli/faq#c19
You SHOULD but you don't have to. They say you can, but it's not recommended, you can try running CoolBits which makes your graphics cards run at the smaller of the two different memories. Why are you going for SLI? Are you planning on buying another 560 Ti or do you already have one?