I doubt if it happened frequently Richard. But as they say there is a prototype for everything so don't be surprised if a pic turns up showing just that event happening. Heavy passenger trains carrying people on holiday to the coast resorts of Southern and South Western England often needed double heading on the GWR so it is likely the Southern did the same on occasions. This would be in the post war (1946-1960) kind of period before many had cars and pre Beeching.
I should have mentioned that I am Great Western afficionado, the Southern (who ran a lot of electric multiple unit trains in the commuter areas of SE England) never fired my enthusiasm. The nearest they came to here was 30 miles North at Exeter or 30 miles West at Plymouth.