Yeah, I know, Another "Nice Pair" blip

A little unoriginal I know but its peeing it down outside (still) and im trying to learn a little more about flash!

I underexposed this shot which created a darker background than I would have like but I think ive managed to save it in photoshop by using an overexposed RAW for the background!

Does anyone know the correct way to bleach out the background? Im guessing an off camera flash aimed at the background and another flash aimed at the subject?