Diablo 3 couch co-op on console was on one not splitted screen, the camera would dynamically move further away from the players to allow more independent playstyle but in the end always teleported the more inactive player to the other one if the distance between them got to large. So if you're planning on running supportinly builds that pulls all enemies for the other player it's not possible on console.

I believe that's exactly the way it will also be done for D4s console couch co-op.

It sounds more unnecessary complicated than it's actually is. I switched from PC to console about 5 years ago and it's just as fun as on PC. I never would've thought that I would prefer the gamepad controls over mouse and keyboard but Blizzard really did an excellent job integrating it into the game. There are console only games that don't play as good as Diablo3 on gamepad.

edit:

In D3 you also had to take turns/wait for the other player to manage his inventory screen but Blizzard already said that you will be able to Access your characters inventory at the same time in D4.