I'm not sure the sales would be as amplified as you believe they would be by a bi-annual release system for both machines. if NSMB DS had had a sequel released 2 years after it, NSMB DS would have nearly stopped selling at that point. After 2 years on the market, it had sold between 13 million and 14 million. Say the total for the game would be 17 million when all is said and done. That's a bit over half of what NSMB DS seems to end up with.
Now, is it likely to assume that as many people would purchase the sequel? If Nintendo's other franchises are anything to go by, no, it isn't. Every single main-line sequel on a system has sold less than the previous one did in their main franchises. It's true for 3D Mario, Zelda, Pokemon, Metroid and Big Brain Academy, as well as sort of for Wii Fit and Kirby. Nintendo's sequels on the same system simply don't sell as much as the first one did.
Which means you're looking at lower sales for the two 2D Super Marios you're going to release on the system. Possibly a lot lower, possibly a bit. Still, you're not going to end anywhere near having 3 30-million selling 2D Marios. The grand total of the 3 wll obviously exceed 30 million - and probably 40 - but I seriously doubt it's going to be the gigantic increase over a single release you're making it out to be.
One of the things that we've seen very clearly with the 7th gen, is that Nintendo sequels on the same system are sales-wise misses.
