They both have to be timed the same as all other scenes, since another set of vehicles will be coming in X seconds. X likely being less than 30. Unless the elevator scene is only half X, and there is some show after it. No way that will be the case with the ending. Especially with the crazy rumors people have built it up to being, if anything, I could see that section being 2X long, meaning there would have to then be 8 down elevators.
Perhaps the up elevators are really fast, but seeing as how the cars must park perfectly to initialize the locking sequence, THEN the elevator can move, I see no way there will 2 up elevators, each holding one car.
Unless that is the big issue with the design, meaning they should have hired me, instead of the engineers they currently have.