This may not be pertinent but I would think that there are a few important things you left out...
1.- Budget you have to purchase the two cards
2.- System specs of the Rigs you have now or will built for your brother
3.- Is he playing Online or more like Solo Campaigns... what Genre does he plays the most?
Does he (or you) want two cards because you think or heard its better or because you know for a fact that it would be better for gaming? just asking. Personally, I have gone the SLI way and I found ZERO progress with them. People tell me that it was because I did it when Games did not worked well with SLI and today they do.
Then I ask myself... I only play Shooters and most of them are Unoptimized/Bad coded Console ports so do I really want to believe they are programmed to take advantage of SLI/Xfire? if my system specs are on the low side, would I do better with 1 card than perhaps bottlenecking the system with two? See... more questions than answers at the moment