No. Actually tends to decrease the sales because the console owners would have more choices of games to buy.
Really? I mean, what if Halo was multiplat? Would it necessarily hurt overall sales of the game? You'd have it on Xbone and it would sell what it has sold as of now, along with PS4 sales. If Nintendo would go 3rd party, there'd be 2 consoles to choose from and Nintendo fans, just like the Sega fans, would have no other choice but to buy one of the two and purchase Nintendo's content. I don't think it could hurt more than it helps. Sonic Adventure sold worse on Dreamcast than say Sonic heroes which was on 3 platforms. That's the kind of situation I'm proposing here.