So what, like...Well if you consider New Super Luigi U a completely new game from NSMBU then sure. But you're failing to understand why it would be great to also be able to download it straight into your copy of MK8Deluxe so that when you're online you can play all your MK8Deluxe stages and the new stages all mixed together instead of being restricted to just the games on one of the titles (the same way how DLC from MK8 was handled online), but having just one of the title's content would also be an option for those that don't want to get the most out of Mario Kart as a platform.
MK8D Part A + MK8D Part B
1/2 of tracks 1/2 of tracks
I mean, it's still split into separate parts. If somebody had Part A and you had Part B, it's just like before - you don't have the tracks, you can't play together.
At least you would probably get them cheaper, but then you'd have to switch between them if you want to play the other tracks.
The way it's split up, they might as well be two separate games because they still wouldn't work together.
I still don't think it's such a good idea. On paper, yeah, but actually trying it?