Outside of unit failures and delay consolidations I haven't seen a deliberate multiple unit in regular daytime service; although one Sunday during the Six Nations, I believe the Scotland match, I worked a 4-car unit (2+2) from NWP to Manchester Piccadilly, swapping over from the rear to the front with my trolley at Hereford, and then back again at Wilmslow, exchanging places with the guard each time.
At Manchester, the rear two carriages were locked out and we ran 2-car only back to CDF from MAN. Cue many confused people pushing buttons on doors even after myself and the guard had repeatedly stated "Front two carriages only!"
When it got to CDF ready to take rugby-goers west the rear 2 carriages were opened again.