Mithrandir said:

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.

I don't think that Luke going around with Red and Vader with Blue for most of the movie is a good idea. But if Vader had both sabres, and Luke force ripped one from him, and it happened to be red, and Luke used that in his rage to beat down Vader...

That would be pretty cool. This is Dark Side Luke. If he happened to grab Vader's dark side sabre... it would be a powerful moment.

Good idea.