The plot was where Hermione Granger brewed an experimental potion to understand Ron Weasley's mind. But ends up with a side effect of amplifying every kind of spell they cast. Understanding the potential use of the potion, she and Ron, share the potion with Ginny and Harry, Neville and Luna, Tonks and Lupin.
Each couple apparently took a kind of specilization in the type of magic they ampified, Ron and Hermione got almost unbreakable shield, Harry and Ginny could pushback Voldemort Occlumency, Neville and Luna could do Divination a lot more accurate than Trelawney, Tonks and Lupin shared their morphing ability.
Also everytime each couple, well, had sex, their amplified magic tends to create a wind vortex which wrecked up the room they were in.
Can't remember if it was a finished fanfic or not. But I remember that it's a quite long, more than 10 chapter, fanfic.