Meat Locker // Drowned Diner

An ability that triggers "when you unlock this door" triggers when that door becomes unlocked. This can happen one of two ways: (1) the door becomes unlocked on the battlefield or (2) the door becomes unlocked as the Room enters the battlefield because you cast the corresponding half. In the latter case, since the door becoming unlocked is what causes the ability to trigger, effects that cause abilities that trigger when a permanent enters to trigger an additional time (such as that of Panharmonicon) won't apply.

While on the battlefield, a Room's characteristics are a combination of the characteristics of its unlocked doors. For example, if Bottomless Pool // Locker Room is on the battlefield with both doors unlocked, its names are Bottomless Pool and Locker Room, its mana value is 6, it's a Room Enchantment, and it has the abilities in each door's text box.

Any time you have priority during a main phase of your turn and the stack is empty, you may pay the mana cost of a locked door (also called its "unlock cost"). That door becomes unlocked. This is a special action. It doesn't use the stack and can't be responded to.