A single 680 performs better than a 1050ti. I'm seeing 680 go for $160+ and a 1050ti new is $145. If you can find 2 680 for that price, that's a very good deal and I'd wonder why it's so low.
In the right circumstances the 680s would perform better but at ridiculous power consumption. If you're at 1080p/60 and the ti is the only thing in your budget I'd stick with that. Otherwise you'll probably need a new psu as well.
A single 680 performs better than a 1050ti. I'm seeing 680 go for $160+ and a 1050ti new is $145. If you can find 2 680 for that price, that's a very good deal and I'd wonder why it's so low.