Like title says, I think something like this should theoretically be possible, the Wii had a little known homebrew app known as GB4GB (https://gbatemp.net/threads/gb4gb-for-the-wii-released.282279/), capable of emulating 4 GB(C)s at once, with the top 2 and bottom 2 capable of linking up. It wasn't perfect, and I think it was abandoned after only its initial release. All the same it was a proof of concept that proved even the Wii capable of such a concept. With the Wii U having more power and being better built for such a concept with one system emulated on the TV and the other on the Gamepad, it could be the perfect homebrew use of such hardware. Maybe up to 4 instances could be emulated at the same time, but I think aiming for 2 GBAs at once in such a set up would be the ideal thing to first aim for. This would be so great in allowing us to experience so many great classic Gameboy (Advance) multiplayer modes that are pretty much locked away on modern hardware.
Of course, I don't know what the hell I'd be doing, so I'm not here to say I'm doing this, but I wanted to float the idea out there because I love the idea, and think it'd be a great unique utilization of the Wii U's Gamepad in conjunction to what different is being displayed on the TV, because as far I've seen, I've noticed very little of the Wii U homebrew scene try to take advantage of this unique nature of the Wii U, asymetric gameplay I believe is how they were marketing the idea of 2 different things happening with the Wii U and on the Gamepad. Would be really neat to see some of that stuff happen in the homebrew scene for the system. Of course I know it's not a super active scene, but I can't help but notice the missed potential we have lying here.
Of course, I don't know what the hell I'd be doing, so I'm not here to say I'm doing this, but I wanted to float the idea out there because I love the idea, and think it'd be a great unique utilization of the Wii U's Gamepad in conjunction to what different is being displayed on the TV, because as far I've seen, I've noticed very little of the Wii U homebrew scene try to take advantage of this unique nature of the Wii U, asymetric gameplay I believe is how they were marketing the idea of 2 different things happening with the Wii U and on the Gamepad. Would be really neat to see some of that stuff happen in the homebrew scene for the system. Of course I know it's not a super active scene, but I can't help but notice the missed potential we have lying here.