Hello there, fellow Skyrim nerd here.
I'm pretty sure the cards are just fine, it just so happens that Skyrim+ENB doesn't really like SLI-CF setups. You really need to do a lot of things to get it work and there is a good chance that you never will.
But I'm also sure that you shouldn't be getting that low FPS, try to change your SLI profile for Skyrim. Also make sure that you have Temporal AA disabled in enblocal.ini.
I would advise you to take this topic to ENB's own forums but I don't think even Boris could help you since he already stated a lot of times that he never meant to make ENB compatible with multi GPUs.
His own words:
"Folks, sli and crossfire are hacks, they have several methods to draw screen and because i can't test myself - do not support them. Low performance happen, because videodriver can't decide which of my data is independent between frames, so transfering them to another videocard, this is how i understand the problem. I tried to make changes like was said in sli programming guides, but that did nothing actually. Wanna have good performance in general? Configure mod properly. In 4k or supersampling you don't need full size ssao at highest quality, as example, you just don't see difference."