#Brachylog, 17 bytes
'(s[A:B]+~m[?:I])
or
'(sSl2,S+~m[?:I])
or
:{s[A:B]+.}f:?rx?
or even
:{s[A:B]+.}fF:?xF
Can't get less than 17 bytes…
###Explanation
The first two programs basically say “Prove that no subset of 2 elements of the input exists such that its sum is in the Input”.
The last two programs say “Find all sums of subsets of 2 elements of the input, none of the elements of that list are in the input”.