Q: How to enable multiplayer?
A: Go to options and press the player 2 toggle button, then go to the main menu and start a new game.
Starting a new game is important because it lets player 2 create their character.
Once player 2 is created, the player 2 toggle button can be used freely to swap between the singleplayer and multiplayer.

Q: We don't have controllers, can we still play together?
A: For the best experience it's recommended to have a controller for one of the players, but the multiplayer can be played without any controllers.

Q: We want to try player-versus-player combat. What do we need?
A: The only requirement is that the players have to belong to different factions.
The easiest way to do that is for one of the players to equip a pink cape which can be purchased from Chaos agent NPC in Moonrise Desert.

Q: We don't want to fight each other, how do we stop dealing damage to our characters?
A: The easiest way to make sure players can't damage each other is to stay faction-neutral: Don't have any items with "Changes faction to..." equipped.
Alternatively, both players can equip items of the same faction.

Q: Can controls be changed? How to enable controllers?
A: To customize controls and enable controllers, open options and go to "Player 1 controls" and "Player 2 controls".

Q: Will there be online multiplayer?
A: There already kind of is thanks to Steam's Remote Play Together.
Besides that, no, there won't be built-in online multiplayer.