When you think about it that can be a very awesome feature but super duper difficult. Lets think about the games that have done it before and well, in the old 2D games. There is allot but lets take Sonic and Kirby Super Star for instance. Both had some sort of system where if the second player got lost he/she could catch up instantly via star warping or auto flying.
What was going to happen in Conker's Quest was 2 player split screen, with a Squirl Character and an owl character. With the second player as the flying owl it would be easy for the second player to keep up during platform elements. or getting across ground absticles period. Same thing with Tails only thing is, If Sonic was too fast and Tails didn't know the layout of the game he would be left behind.
Mario and Luigi are super duper difficult in the 3D realm, only because both are limited to more or less of same abilities. So both players would need to be pretty good to get around those really difficult platforming sections together. And like you said Mop its gotta be the same for one player and 2. Or maybe no, what if when both bros are present things are slightly different. Also in such a game split screen might make things allittle more difficult in really precise platforming moments. So I say one big screen that pans out if the the characters are too far apart.
I'm still convinced there would need to be some kind of catch up system for the second player, if the camera follows the first player soley.
But what excited me most is possibly having Mario and Luigi performing their team moves. Like a super triple jump where they lock arms and flip around each other. Or all that zany stuff in the Mario and Luigi games. Imagine racon/cape/wing hat Mario and Luigi flying mid air together to perform awesome Mario like stunts. There would be some lock on action and once Mario and Luigi lock on/team up you can perform awesome moves.
I've dreamed about this for a while. If I ever talk to that Matt from IGN again, im going to bring that up to him and see what he thinks. If he's interested in it, he might bring it up to Miyamoto. I know Miyamoto likes co-op Mario, but what does he really dream of making a possibility?