This was another one of Bulleid's ideas that need to be put into context.
Post War, the Southern was facing sudden critical overcrowding and they had worked out the expense of introducing 10-coach trains. This would have involved extending platforms with consequent major changes in track layouts and signalling. It was going to cost 10's of millions of pounds at 1948 prices, and take years to implement. (= Billions in today's prices??)
So the Double Decker was
exactly the right technical solution on paper for speed of introduction and economy. But it didn’t work for many well documented reasons. At least he didn’t build 100s of them before discovering the problem.
This is what fascinates me about Bulleid. So many of his ideas were absolutely right on paper, but couldn’t
quite be delivered, so the idea ended up as failures.