The crash between a Ford Ka and a Ford Focus happened on the A1 near Torness power station in Dunbar, East Lothian, at about 7.40pm on Thursday January 9.
The Focus driver, 23-year-old David Forsyth from Invergordon, Highland, was taken to hospital after the collision but has since died, police said.
Dawn Morris, the 21-year-old driver of the Ford Ka, was pronounced dead at the scene.
Her rear seat passenger, 21-year-old Claire Jamieson, died in hospital last week after suffering serious injuries.
Both women were from the Cowdenbeath area of Fife.
Two other 20-year-old women, who were car passengers, were injured in the crash.
An investigation into the full circumstances surrounding the incident is continuing, Police Scotland said.
The road was closed for about six hours while officers carried out investigation work.
Any motorists who witnessed the crash, or who have information that may help the probe, is being asked to contact police immediately.
Police Scotland can be contacted on 101.