A 22-year-old man and 19-year-old woman were killed when their car careered off the A38 in Somerset and crashed into a tree.
Avon and Somerset Police said two cars were travelling in the same direction when the accident happened, near Thurloxton, on Sunday morning.
A force spokesman said: "At around 10.30am two cars, a Peugeot RCZ and Vauxhall Astra were travelling along the road from the Bridgwater direction towards Taunton.
"The Peugeot allowed the Astra to pass and for reasons unknown the Astra lost control and struck a tree. Both the driver and the front seat passenger died at the scene."
Two people were arrested but later released without charge. No further action will be taken against them. Neither of the victims have been named.
"Officers investigating this incident would now like to speak to anyone who was in the area and witnessed this collision," the spokesman added.
Devon and Cornwall Police are also appealing for witnesses after a motorcyclist suffered life threatening injuries in a crash in North Devon on Saturday afternoon.
The scene, on the A3072, at De Bath Cross, North Tawton, was closed to traffic for nine hours, following the crash, between a green Vauxhall Zafira and a Yamaha motorcycle.
The rider was airlifted to Derriford Hospital in Plymouth where he remains in a critical, but stable, condition.