Valheim doesn't support mods officially yet. They might as the game gets closer to version 1.0. So no, there is no Steam Workshop for this. Try Thunderstore.io.

Of course, all mods might break Valheim after every major patch. So if you don't want to risk that, there is no way to enable devcommands in multiplayer.

What you can alternatively do, is have one player spawn a bunch of things the 2nd player can just pick up and use. That way, while it's not full creative for everyone, at least, it's less work for the 2nd player.