Mirror Room // Fractured Realm
Two doors, two costs, and a sequencing puzzle wedged between them. The cheap front half staples a clone to a permanent that stays on the battlefield after it fires: unlocking the door is itself a triggered ability rather than a spell that resolves off the stack and leaves, so the enchantment lingers where a one-shot copy spell would not. The back half is a different animal. Doubling every triggered ability of your permanents is one of the highest ceilings blue is allowed to reach, the sort of recurring multiplier normally fenced behind dedicated blink or engine shells. Both halves are strictly sorcery-speed, since a door only opens on your own turn with its full cost paid at once.
The interaction between the two is what rewards a precise order of operations. Unlock Fractured Realm first, and the front door's "when you unlock this door" is a triggered ability of a permanent you control, so the doubler catches it: unlocking Mirror Room then fires twice and mints two Reflection copies instead of one. Reverse the order and the unlock trigger has already resolved before the doubler exists, leaving you a single token and no interaction at all. Neither door needs the other to justify its cost, but the payoff is engineered for a player who intends to open both and knows which goes first.
