I could be wrong but I suspect that this concept was born out of experimentation with DS-Wii connectivity ...
I wouldn't be surprised if Nintendo had dozens of ideas for games, and for interactions within games they were already developing, and ignored these because they were unconvinced that people would use the systems together; or because some technical limitation between the two platforms made it (nearly) impossible to create their vision.
On the topic of multiple tablet-controllers per console, with Nintendo's track record on support for local multiplayer I would expect that the system could support at least 2 and probably 4 tablet controllers at the same time. With that said, I suspect that the more tablet controllers are connected the less Nintendo would display to screen; and with 4 or more tablets it would probably only be a simple menu or map displayed to this screen.







