I know that Astroneer allows up to 4 players to play online together in a single lobby. Is it possible for two players to play together locally on the same machine?
You are correct about the online multiplayer but this game is online only. On consoles, the game struggles to run as it is. Imagine having two separate players in the same system, it would explode.
There is no local multiplayer, online is the only way to play. There may be some unofficial mods for the PC to add extra features like this but the core version of the game does not support it.