After running a Crossfire setup, I would highly recommend not going the dual card route. There are so many games that do not support it and the ones that do usually don't scale very well, in some games the single card beats out dual cards. In a lot of games you do not see the benefits of SLI/Crossfire until you are past 2k resolution as well. You are much better off buying one good card than trying to use 2 slower ones.
https://www.techpowerup.com/reviews/..._GTX_1080_SLI/