The incident happened at 10.40pm on Friday (February 16) on the Watling Street coastbound carriageway between the A296 and Ebbsfleet junctions.
Three cars were involved, a black Ford Fiesta, a blue Audi S3 and a blue MG HS.
A passenger in the Fiesta was airlifted to a London hospital with serious injuries.
The driver and a second passenger were also treated for injuries which are not believed to be serious.
The driver of the blue Audi S3 had left the scene before police arrived.
Investigations continue to find him and his female passenger.
Matthew Sparkes and Kitty O’Brien are wanted in connection with the crash.
They are both described as white, in their 20s, and were last seen walking away from the scene on foot.
The man is described as being slim with short blonde hair and the woman is of medium build with shoulder length blonde hair.
Kent Police is urging them to come forward and anyone who witnessed the crash or has CCTV or dashcam footage to contact them.
Anyone with information on their whereabouts should call 999, quoting reference 46/26784/24.
A spokesperson for Kent Police said: “Investigators continue to carry out enquiries as they search for a motorist involved in a serious collision on the A2 near Dartford.
“Kent Police’s Serious Collision Investigation Unit (SCIU) is urging them to come forward and anyone who witnessed the collision or has CCTV or dashcam footage to contact them.
“Matthew Sparkes and Kitty O’Brien are wanted in connection with a serious collision on the A2 near Dartford on Friday 16 February, where the driver and passenger are reported to have left the scene.
“Anyone with information on their whereabouts should call 999, quoting reference 46/26784/24.”
