I remember feeling that they also felt like a real couple and was impressed by that when I was watching the first season. Then I wondered if maybe my affection for the show was "tainting" my opinion too much, until around Christmas time. My buddy shared a Trixie and Katya Watch episode screening the first season. They had their usual jaded attitude, but also thought the show was cute. On the last episode they made the same statement, they felt like a real couple. I felt validated.
Pretty cool for 2 actors that young, one of whom was doing his first professional gig.
Slightly related (and forgive my text wall), another pair in Only Friends channel that same kind of energy. The guys playing Top & Mew (Force & Book) have been best friends for 16 years, and that comes across at the comfort level they bring to their performances. They have less acting experience than Khao & First, but their chemistry has been on point since their first series. They actually have always played as an onscreen couple for all three series they've been in.