A nice co-op-but-not puzzle game. Bouncing blocks (and each other) on top of your head was an unexpected mechanic which I thought was a bug at first but proved to be vital to success. :)
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Thank you! That is not properly a mechanic, as you can do the same jumping while carrying the box, but we left it in to make it easier to launch blocks up (As the alternative is to release the grip while jumping, but requires good timing and is hard to explain with level design alone).
Anyway these little alternatives made it much easier to avoid player frustration without making a full tutorial to explain the intended mechanics.