How did the return "mirror switch" work in the episode Mirror Mirror?

Question

In the episode Mirror Mirror the landing party get sent to a mirror universe during a transporter malfunction while beaming back to the Enterprise. They conclude that they must have switched places with their "mirrors" who were also using the transporter. Eventually Scotty (the man is a magician) figures out how to send them back using the transporter.

When they appear back on the USS Enterprise they are on the transporter pad. But doesn't this imply that the "mirrors" would also have to be in the process of using the transporter for this switch to occur again? And if so, how would Spock on the USS Enterprise know when to place the "mirrors" on the transporter pad (he had them placed in a holding cell)?

Answer 1

Out-of-universe - It's just part of the story not directly related to the main point of the plot. Try not to over think it ;-)

In universe = in the mirror universe Scotty had to make heavy use of the computer to calculate how to get back. Spock in the real universe probably had to do the same thing. In the mirror universe Sulu and Spock delayed the party while they were trying to leave. In the real universe the mirror captain and crew probably also delayed the real Spock taking them to the transporter (including a probable delay in Spock identifying the mirror crew in the first place). End result = they transport back at the same time purely by coincidence.

In later series (DS9) we see that people can cross over either way without exchanging places with their counterparts.

Answer 2

The mirror universe team just sort of disappeared when Scotty managed to send our team back. No explanation is given, but our Spock theorizes that they were somehow sent back when our team arrived back on our side.

A quote from the Memory Alpha article on Mirror, Mirror, bolding mine:

On the bridge, as the Enterprise leaves orbit, Kirk asks how Spock detected their counterparts so quickly. Spock tells Kirk that as civilized men, they were able to impersonate barbarians much easier than it was for barbarians to imitate civilized men. Spock speculates that the counterparts returned to their their Enterprise at the same time the landing party arrived home.

I would theorize that the original transporter accident somehow linked the two groups, so that they couldn't be in the same universe simultaneously, like they could be in later series.