I think it was supposed to be asymmetric play, with off-screen being a big additional selling point. If you're wondering what the difference between those two is, asymmetric play was supposed to be like motion controls where it was the hook that leads people to "get" the console, and off-screen play was more like something like backwards compatibility where it's a big omnipresent additional bonus that won't appeal to everyone who buys the console but will be a big deal for some.
As for how they expected asymmetric to be the central hook and selling point of the console, look at how Nintendo Land utilizes the two screens in various different ways. I think Nintendo expected developers to do similar things in their own games, but almost none of them did. They also expected it to be something that clicks for a lot of mainstream consumers, but it sort of did the inverse, where the only people who thought it was cool and saw value in it were their own base of enthusiasts.