This information only applies to The Escapists 2.
Multiplayer Doors are special doors that can only be opened with 2 or more players, the door may be opened by one player holding a metal garage door or activating a keycard switch that the other player has to go in. The multiplayer areas usually have a desk containing valuables for multiplayer escapes, but some (like Center Perks 2.0) have the multiplayer escape areas in them.
You can only go through the keycard doors with a Fake (color) Keycard that is obtained by knocking out the guard who has the color of the keycard. Then, make a keycard mould with the keycard, a wire and circuit board, then craft a Fake Keycard with the mould and molten plastic. Just like molding a Key, you must place the keycard back into the guard's inventory before the medics take them away, or you'll set off a lockdown and get sent to solitary. (Keycard moulds require 50 intellect to craft, while Fake Keycards require 60.) One player, the one with the fake keycard, will press a button near the door. As long as the player stays holding the button, the door stays open. The other player(s) enter and press an interior button to hold the door open for the first to enter.
To open a 'garage door' or 'sliding door'-type multiplayer door, one player will have to go interact with a side of the door where it says "hold open". By alternating between two keys/controls (similarly to certain workouts) the door will be held open, allowing other players to interact with the small opening to pass through. Similar to the above keycard doors, a button inside the area allows others to enter.