ch3mn3y

i've ran xubuntu on absolute garbage, i dont see it being an issue.

i have a computer by the TV just running xubuntu (because lightweight enough) with a wireless keyboard that has some application keys (you know, the ones that are like "open mail", and "open music"), and i've just set kodi as my default music application, and retropie as my default mail application.
 
result: if i press the music button, kodi pops up, if i then close kodi and press the mail button, retropie pops up. linux is fairly "dumb" about this, so you can pretty much assign any application for most of the "application" buttons.

While what you're looking to do is not really a common use case so there isn't a lot of documentation about it. You'd get more performance by selling both cards and upgrading to a single better card. Especially if you pick up something used rather than buy new. 
 
If you CF a 580 and a 570, performance is going to be limited by the 570. Coupled with how few games actually support crossfire and adding on top of that how many of those games run CF well......it's best just to find the time and find the money to upgrade to a faster single card.