It's an OW encounter that states the investigator looks into a strange and distorted mirror and the investigator in the OW can exchange places with another investigator.
The investigator in the OW had been drawn in without sufficient clues, so I switched places with one that did. Does that second investigator have an encounter also, or is it considered all part of the first investigator's encounter?
Would it depend on turn order?
All you are doing is changing their physical locations. They would still have an encounter in turn order. Or if they already had one, play would just continue on.