Why would you go to all the trouble of having the same interface/OS/games for a handheld and home device, but then cause headaches for developers by making one device dual screened and the home device is going to be primarily single screen (the TV, and sorry the Wii U's failure largely illustrates the stupidity of that concept, unless human's grow a third eye sitting on a couch they can only view one screen at a time).
Who even likes the dual screen setup? Like honestly which game in the last 5 years has used it in a profoundly interesting way? I remember I had my little cousins come over and I gave one a tablet to play with and the other one got my 3DS and the one that got my 3DS looked like I had asked him to eat his vegetables. 3D is old hat too, that was something that was cool maybe 6 years ago when Avatar came out, now it's losing its appeal.
Even kids don't like the dual screen setup. One big, vibrant full touch panel is more intuitive and allows developers to focus all their energy on one display, rather than 90% of games having a secondary display that does nothing but show the map and eat up battery needlessly.
If Nintendo was ever going to start start taking some design risks now is the time. In 5-6 years honestly their handheld userbase could be easily down to 30-40 million if they keep sticking to designs/principals that are over a decade old now. The world has moved on and is leaving them behind.
The dual screen design just screams "hey! Remember when Britney Spears was the biggest pop act, George Bush was president, and Sopranos was the best show on TV? Yeah we're the company stuck in a decade+ ago". It's a bad idea, the gains don't outweight the optics, people will see even an 8 inch iPad mini with a much higher resolution screen and then look at that Nintendo handheld with two smaller screens and chuckle at it.
