A new update on the Wasteland 3 Fig page discusses how the game’s multiplayer will function.
Players will be able to start the campaign in multiplayer with a friend but the campaign will be tied to the two players. Should one of the players drop out, they can’t be replaced by another friend. Instead, a single player campaign can “spin off” from the original multiplayer campaign.
Both players won’t be able to have the same NPC companions either, but companions can be moved between each players’ squads. Players and their squads can also separate and go off on their own. The game is asynchronous so if one player is offline while the other is playing, any actions taken will be reflected in the other players’ game when the come back online.
To prevent the game campaign between the players going completely out of sync, both players will need to be online at “key narrative moments and missions”. Both players will need to be present to make key decisions. InXile says this happens “infrequently” so there will be hours of play and side missions available to tackle should one player not be online.
The full update gives an example scenario and it’s worth a read of you’re keeping an eye on the game.