There's a part of the duel when Vader has Luke's sabre. He's got his own red sabre in a hand, but there was some mock up that showed him handing actually both his sabre and luke's both of them turned on. I think this could be more menacing; and what's more, perhaps when Luke attacks Vader in anger, he could grab Vaders lightsabre and defeat his father with that weapon just as a way to increase that feeling of "everything's going bad" you say you wanna give to the movie.

In the PT we've seen plenty of duels with people holding two weapons. ROTJ's duel had its "new toy" by being the only duel in the OT with music, but even in Ady's revisited series, it has lost that status. So perhaps having Vader (only for a while) with two sabres could make it more threatening and enhance the scene a little; and in the end, when Luke screams "never!" he'd start to fight with a red sabre, which is exactly what you're saying here.