How the Wii U and 3DS VC and eShops should work

This one is going to be a quickie.

I am a blatant Nintendo fanboy, but even I have to admit there are some things that are done exceptionally wrong.  The fact that you can't use a 3DS to work as a stand-in for the gamepad for some games is kind of annoying.  The fact that you can't really use two gamepads or more for games... can you imagine how much more accessible NSMBU would have been if two people could have played the game and not been restricted to the same screen?

That's not what I'm going to talk about today.  Rather, it is the buttfungled mess that Nintendo has that they're trying to pass off as their eShop, VC, etc.

There is absolutely no reason that we shouldn't be able to play the same game across the board if its available on both systems (with only one purchase!).  But further than that, there's also no reason that most of the games that are available on one system shouldn't be available on the other.  I understand that there may be some worries that you'd cannibalize your sales, but I have an alternative that would work fine:

If you don't want to allow someone to purchase a game w/o a 3DS or Wii U, you simply link each system to each other.  You still have to purchase the game on the system its released for, but once the transaction is made, you can play it on the linked system as well. 

Why has this not been done yet?  I would be greatly excited to hear about something like this at E3.