Up to four wizards can share a run. Everyone plays their own character, keeps their own loot, and the world scales up a little for each extra player.

On one computer (couch co-op)

• Plug in another controller and press Start during a run to join, then pick a save slot for that player (an empty slot makes a new character).
• To decide which device belongs to which player — including the keyboard and mouse, which count as one — open Options → Controls → Players.

Online

• One player picks Host Game (optionally with a password) and chooses a save.
• Everyone else picks Join Game, enters the host's address (port 27511), their name and the password, and brings one of their own characters.
• The host may need to forward port 27511 on their router to play over the internet; on the same network it just works.

When someone goes down

A fallen wizard is downed, not dead: they can crawl, and anyone standing next to them can hold Interact to revive them at half health. If nobody reaches them in time they lose their socketed spell modifiers and respawn near the group a few seconds later — and if everyone goes down at once, the world reshuffles.