Yes, this has to be played by two people. It is somewhat playable with one person, but as you say, the starting weapon is not designed for that. We wanted to focus on the two player experience, so there was no time to polish it for one person. We will see if it makes sense anyway, since we want to have the game design in a way so it is cooperative. :)
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But we were thinking about having two players on keyboard but there was no time for that either since we are using the new input system in Unity and the default input manager does not allow to assign one device to more people. Hopefully we will fix this in the future since it would allow one person to control both players more easily (like in the Brothers game) and make the singleplayer experience possible and interesting while not breaking the coop one.