Police determined that their Honda bike had struck a tree planter on the stairway to a chapel, about one kilometre from the main road.
Monks rising for their morning alms rounds found the victims at the temple in Moo 3 village, Mae Sai district.
The dead were identified as Kiartisak Srisahas, 45, and Pratchaya Saehuang, 47.
A physician decided they had died from severe head injuries about six hours before their bodies were found.
One of the monks, Phra Suriya Naijai, told police he’d heard a loud noise, as if a large tree branch had broken off, at 1 am and dogs in the temple began barking, but no one checked to see what had happened.