You can't dual mine Skein+Eth. Since you have two cards, you could use one to mine Skein, and the other to mine Eth.
Technically, the one that you mine Eth could dual mine one of the compatible coins like Lbry, Pascal, Decred, or Sia. That would mean you are actually mining 3 coins with two cards.
Nvidia cards are not great at dual mining though. It's probably best to stick with one algorithm/coin, and use both the mine the crap out of it. The 1080ti is a great performer on certain types of algorithms. Check whattomine.com to see what some of those are.
1080/1080Ti are very good in dual mining, only their ETH hashrate is not good which is compensated by much more sia/lbry/dcr/pascTechnically, the one that you mine Eth could dual mine one of the compatible coins like Lbry, Pascal, Decred, or Sia. That would mean you are actually mining 3 coins with two cards.
Nvidia cards are not great at dual mining though. It's probably best to stick with one algorithm/coin, and use both the mine the crap out of it. The 1080ti is a great performer on certain types of algorithms. Check whattomine.com to see what some of those are.
Anyway there are a lot of other algos where they truly shine.