No its not worth it. SLI is barely supported by any modern title, and even those that properly support it can not leverage both GPU's evenly. You aren't buying double the performance, you are buying anything between 20 and 80% depending on whether the game you are playing actually supports SLI. SLI also disables certain AA features within games since its not possible to render certain forms of AA when the render tasks are split between two GPU's.
SLI introduces problems of its own especially in titles that do not support it, not least of which are frame timing issues. You may have an x370 F motherboard but you can't run these cards at full x16 at the same time because there are insufficient CPU PCIe datalanes to do the job so you are already bottle-necking both in terms of bandwidth. Buying a single GTX 1080Ti is a much smarter choice at least you;ll be able to fully utilize this GPU rather than partially using two 1070's that together introduce issues, are barely supported and neither of which can reach the performance of a 1080Ti.
There is almost no one here who is going to tell you otherwise, it isn't a smart move, its not justifiable by any metric, a single higher performing card will be a better choice than two lower tier cards in a barely supported configuration end of story.