Ambidangerous

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Ambidangerous is an indie top-down shooter that challenges your ambidexterity. Simultaneously control two elite soldiers as you fight your way through four randomized areas, five unique bosses, and a dozen enemy types. Take the challenge alone or share the controller with a friend for co-op play.

Yep, you can play couch co-op with just one controller, at the same time. Just snuggle up to a friend and each of you grip one side---the controls are mapped so that each hand works independent of the other. Or you can use two controllers. But a controller is required; mouse and keyboard are not supported.