Surprised to hear that, Tentacles, and many thanks for the update, sejo.
When March' went from Mediator to NODE-M in the early 1990s, both the ATCOs were allowed to use the frequency, and this continued until at least 2011, after the move north.
Under the Mediator system, there was usually only one ATCO per freq', as the Chief Sector Controllers didn't wear headsets.
When EGCC got very busy in the late 1980s there was a facility to 'man-and-boy' each position, where another bod would plug in to help, in addition to the CSCs.
I think P and E was introduced at Atlantic House, round about 1979.......I picked up a copy of 'Airway' at my final selection board that year, and remember reading about it.
I thought when LATCC moved to Swanwick, with the 3 bods on each 'banana', it was a similar set-up. ScATCC had evolved largely to a one ATCO per frequency system by the time nPC came on-stream.