The two men who died have not been named by police but were aged 22 and 24 and both were from east London.
Two other passengers in the same car were also hurt and taken to St Mary's Hospital in Paddington, central London, with "serious injuries", Hertfordshire Police said.
Four men travelling in the other BMW also needed hospital treatment for their injuries.
The accident, which happened between junctions five and four, resulted in the closure of five junctions of the motorway in both directions, and the southbound stretch was shut for several hours.
Hertfordshire Police have appealed for witnesses to the crash or the events that led up to it to contact them.