I have tried searching this but must not have found the perfect combination of terms.
It's a pair of fics set in Season 7 at the height of the battle with the first. I think the first is more or less a Porn w/ Plot and the sequel is a bit longer and deals with the fallout with everyone else.
Basically, Spike tells Buffy that they can beat the First if they mate, but only if they are truly in love. If they are truly in love then each of them double their powers by inheriting each other's strengths; the opposite happens if one of them is not true in their feelings.
Thanks guys! | 
