Riding two yoshis with different abilities in mario & luigi controlled with x, y button, and involved in the attack.
It would work for a minigame, but as a gameplay mechanic of it's own it would be hard to implement, especially since Mario & Luigi in these games get blown up to (whatever place that exists) in each game, and since you would have to worry about him, things would just get confusing, especially considering how Maps and Puzzles in these games requires you to use the special abilities of the Mario Bros, and since you won't probably have all of the abilities available while riding Yoshi, it would be a little tedious to "Ride, Run to "X" place, reach "x" place, dismount, change ability, use it with a wall or something, unlock, ride again, go on" i think the normal things we have are more than enough.