All puzzles are designed with the single-player experience in mind first - co-op naturally follows. You simply control one character at a time with a hotkey to switch control. All puzzles are asynchronous, meaning you never have to do anything at the same time with both characters.
We tried it, and basically designing space for them to have differences in movement causes problems for the other, due to the nature of negative space. You add to one character's space, you take away from the other's space. Besides, I like the fact they're equal in ability :)
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