The Deltics always ran on BOTH engines in service, they needed all their power to maintain the tight schedules on the east coast main line, these frequently required 100mph running. They only ran on one engine when they had breakdown on one of the engines, this allowed them to continue their journey at reduced speed rather than becoming a cripple like other single engined locos.
PS: The twin engines made so much noise that many of their drivers suffered from hearing loss in later life.
