As one data point, as a GLD a few months ago I called and asked AA to "link" PNRs for me and a travel companion (no status). In the same call, I had them move my companion to be adjacent to me (exit row). The flight times were subsequently changed by AA, and we were placed in nonadjacent seats.
The AAgent told me at the time that she would indeed "link" the PNRs, but per info here it seems that there is no such thing, and the fact that the seats were separated is consistent with that.
Am I correct that if my companion and I had been on the same PNR, then our changed seats would still have been adjacent, if space allowed?