The two characters have different abilities that can be used to solve puzzles and enemy encounters either alone or co-op.
We've seen a portion of pre-alpha gameplay accompanied by dev commentary. Spoiler alert: it takes two to play the third entry.
We learn more on why LN3 didn't go "mandatory co-op" like It Takes Two, but still offers a friend pass.
Supermassive Games will bring it to us with co-op in 2024.