#Factor, 120 bytes

I've spent class golfing this and I can't get it shorter.

Translation of: [Julia](http://codegolf.stackexchange.com/a/67876/46231).

    [ 1 swap [a,b] dup append 2 <combinations> [ members ] keep [ first2 coprime? ] filter [ length ] bi@ 2 /i 1 + swap /f ]