I've certainly refused upgrades, it's quite easy at the gate, more hit or miss if one clears before the day of the flight and the other doesn't and you're calling in to try to get your original seat reinstated. (I had the same refused upgrade re-clear 2 more times once during the window, finally got someone on the Premier Line who just upgraded my companion as well since there was R but the PNR was split earlier in the trip and she agreed that if it hadn't have split then we'd have both been upgraded together, this was well before the split PNR issue was fixed).
In my most recent case there were 2 seats available and my wife and I were
#1 &
#2 on the gate list but the seats were not together so I told the gate agent we didn't want them. She said no problem, took us off the list and gave it to an older couple who were
#3 &
#4 . They seemed quite happy to get them and didn't mind being split up for the flight.