Some years ago, my wife and were holidaying in the Dominican Republic, flying with Thomsons. I had requested, and paid for, "Seats together" as my wife is a VERY nervous flyer. We didn't actually get our seats allocated until check in, so I was surprised to see we were "across the aisle", which wasn't too bad I suppose.
On the return leg, I was shocked to see that we were allocated seats that were behind each other. I asked a member of Cabin Crew if that was correct and was told to refer to the Terms and Conditions on the Website when we returned home.
I wrote and complained to Thomsons on our return home and was told the same, which is:
" Seats together may be across an aisle or behind each other. A similar position means the same type of seat (window, aisle etc) as your original selected seats."
Can Forum members tell me if that is universal across all airlines?