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In the Star Trek, Deep Space Nine, episode "Crossover" two members of the DS9 crew crossover to an alternate universe. It is the same universe that Kirk went to many years earlier in the TOS Episode: "Mirror, Mirror."

In this episode the two members crossover going through the wormhole with some engine issues. This causes them to crossover.

In "Mirror, Mirror" Kirk crosses over due to an issue with the transporter.

Aside from the method of crossing over, there is one other major difference in the process, which is that the Kirk from the mirror universe crosses over to the prime universe at the same time as our Kirk goes to the mirror universe. In order for each Kirk to get back to his right universe, they had to recreate the same transporter issue in both universes at the same time.

However in DS9: "Crossover" the two crew members never exchange places.

Why didn't the mirror universe Major Kira and Doctor Bashir crossover at the same time?

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    The TOS episode is actually the only instance of a mirror universe swap taking place - all the mirror universe episodes in DS9 only have one or more people going in on direction with each transit.
    – HorusKol
    Commented May 21, 2015 at 6:47
  • Different crossover method, different rules.
    – Paul D. Waite
    Commented May 21, 2015 at 10:51
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    The first DS9 mirror episode is the only instance where a transporter is not used...
    – HorusKol
    Commented May 21, 2015 at 23:36

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This is discussed in the DS9 episode you mentioned; Crossover.

Kira II indicates that in the Mirror Universe, all transporters were comprehensively redesigned specifically in order to prevent the same accident from occurring. By comparison, transporters in the datum universe weren't changed at all. This evidently allows mono-directional travel when the same kind of accident reoccurs.

KIRA: You know I'm looking for a way back. A transporter was responsible for the first crossover...

KIRA II: And if you had come to me, I would have told you that all the transporters were redesigned afterwards to make sure the same thing could never happen again. It won't work.

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Since there is no in-story explanation, at least as of this writing, one can only assume.

In "Mirror, Mirror," both landing parties were in the process of transporting from the surface of the Halkan planet. The event that caused the two universes to merge was attributed, in part, to the transporter beam. While one could theorize the swap was a universal "safety mechanism" to prevent counterparts from merging, more than likely it was an event duplicated in both universes, with the scrambling of the transporter causing the switch in uniforms. (One could even theorize that only their minds swapped.)

The methods used in subsequent swaps were different, obviously not duplicated between the two planes of existence.

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