Police said the men, aged 20 and 24, were passengers in a southbound Peugeot van which collided with a lorry travelling north, at about 4.30pm. They both died at the scene.
The 63-year-old van driver was taken to the Southern General Hospital in Glasgow, where he is being treated for minor injuries. The driver of the lorry, 24, was not hurt.
Strathclyde Police said the road was closed for more than nine hours.
Constable Alex Smith, from Dumbarton police office, said: "I would urge any witnesses to the crash, or anyone with information that may assist police enquiries, to contact the road policing department on 01389 822000."