This sounds like a cool idea in which you'd be able to play a game on the main console, but there may be certain things you can do on the handheld system as well. That's how I would like to see it at least.
Perhaps the mobile unit is a possible successor to the 3DS that could offer a similar experience to the console. Maybe they'd make so you play a bit on the console and then continue where you left off on the mobile unit.
Any information regarding the NX are all rumors and speculation, as we know next to nothing about it officially other than the fact that it'll revealed on next year's E3.