If you're really a Nintendo gamer, you might remember Zelda: Four Swords on GC. That was the first time they tried this. You went overworld on TV and caves/underworld on your GBA (BEFORE the backlight feature!). It was a mess. And not because of the dark screen on the GBA: it was painful going back and forth because it forced your eyes to adjust in harsh ways. Forcing players to do technical work on a small screen while also switching focus to a large screen... it's the epitome of eye discomfort.
Years later, Nintendo is starved for a new gimmick and.... they go BACK to "CONNECTIVITY", a concept that failed hardcore. At the time, you might have argued that not enough people had GBA's to bring over to your house for a gaming session, but even when including the extra console/controller in the box, it STILL was a bad idea.
At this rate, Nintendo will bring back the POWER GLOVE and call it the latest innovation in motion controls.