So far in Smash duo character have come in 3 flavors
Two characters with the exact same moveset acting in tandem on the stage at the same time.
A dedicated special move to swap between characters
A smaller puppet character with a simplified moveset that can act somewhat independently
oh also Duck hunt if you want to get technical, but the dog and duck together are mechanically very much the same character.
so far I think the main limitation on duo characters on the field on the same time is not so much hardware (although that played a role for Snash 4) but that the two characters would need to be visually cohesive in actual gameplay. Ice Climbers are largely identical in movement and both short so they don’t take up space and it’s easy to follow what they are doing. With Rosalina it’s obvious that she’s the main fighter and that Luma is the puppet due to the size difference.
If Noah and Mio (or any proposed duo character) are both on the stage at the same time, it might a bit visually cluttered as they would be the size of a regular average fighter. This does not make them impossible, but I just want to mention that the visual visual readability aspect is important.