Mac and Linux versions of the games have been confirmed, and with the kickstarter funded there shouldn't be too much of a delay before they are released, compared to the PC version. Currently the editor used some Windows-only components, though.
There will be a demo for the game, so presumably you would be able to use that to play with a friend to see if they are interested in the game (even if it was single player only, they could get a good idea of the gameplay, if not the co-op aspect). Depending on when the demo is made compared to the release finalized, it could theoretically be possible for someone playing the demo to join a game hosted with the full version, and play through part of the game (depending on how the demo is designed and whether it matches the starting section of the full game, etc, etc).
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