In my case what I mean is that a connection between the two data sets (=the two bookings) established, and that this connection is being recognised and processed by the system - as opposed to a note which may or may not be read by a human who then maybe (or not) treats the two reservations like a pair.

Two separate bookings appearing at once together on the online check-in screen requires a system-side connection of the two bookings. That is the kind of link that I was referring to. AF can do that. I have no idea how and in which system (Especially given that Amadeus does not allow it according to the previous post).