However since there aren't any Mistborn left, it would be impossible for someone to be burning both Duralumin and some other metal. And Duralumin by itself would be pretty useless, no matter how far it were boosted. Poor, useless Duralumin mistings. But then, I guess we might get another Mistborn eventually depending on how certain things from Alloy of Law work out in the future.

But anyway, something tells me that stacking them is the sort of thing Sanderson would do, because it would provide a new trick he could play with. But I haven't read anything one way or another. All I've read is that he has plans for Nicrosil mistings to be used during battle, as a way of either supercharging his own people on cue or supercharging enemies at the worst possible moment to screw with them.