To provide a DP on this topic:
I only learned of the "split" trick this year, so all past trips when my SO flew with me, we had two people on the same reservation. In 2021, we were actually 7/7 on upgrades, so it's definitely possible for more than one U spaces to be released at a time. But I do have an observation - not sure if it's valid though - that we have never been upgraded at the airport. Every upgrade that we received were at least a few hours prior, most likely even more than a day. Perhaps the one by one release only happens to gate upgrades? This would be my guess.
Also, it seemed to have nothing to do with seating charts. We've had multiple experiences when we got the upgrade but found no seats next to each other in F.