That is the start of a nice machine indeed, especially with a custom water loop. SLI is more dependent on the game or game engine, some scale better than others. I don't want to turn you away from x99 but if you're strictly gaming have you thought about a Maximus build? Most games still seem to favor higher clock speeds over more cores and the 7700k runs at 4.5GHz, plus the Maximus boards support higher speed ram. With money saved going with Maximus and i7-7700k you'd all but have your 2nd 1080 Ti.
I have the Maximus IX Code and I delidded my 7700k which reduced temps quite a bit and I'm able to play games at 5.3GHz with the cpu cache at 5.0GHz. With a custom water loop the Maximus IX Formula or Extreme are great choices.
Another thing that will determine if you need a 2nd 1080 TI is your monitor, what monitor do you have ?
I don't want to steer you away from X99 but it's something to consider if you're strictly gaming, here is what you can expect after delidding the i7-7700k. Watchdogs 2 is pretty demanding and I only have one ROG Strix 1070 but I'm going to install a Titan X (pascal) in a week or two. 🙂