To do this you need two players and a door that need two players to open. AI will not work. You need an actual second player and two controllers plugged in. The second player must sign in with a seperate account from player one and you need to play in split-screen mode.

The best door is probably the one in Chapter 1: Section 1, you should see it early on and will pass it playing the level normally.

Provided that one of you keeps on interacting with the door (by pressing X), then it will count towards to the Onyx medal score (you need to open the door 200 times to get the Doorman Onyx medal).

It should take 10 minutes.