I did not understand the circumstances completely.

If you had boarding passes that had you sitting together, then I would not have moved. If your boarding passes had you sitting in other seats than the ones you were in, then you would have to move. Another passsenger has requested the seat I have been sitting in a number of times and has been able to show a valid boarding pass for the seat for which I had been given a boarding pass. I believe that if the airline issued two boarding passes for the seat in question, then the passenger who is in the seat first is the rightful owner of the seat for the flight segment in question. It is the airline's responsibility to find the second passenger an acceptable seat.