A double headed hydrant valve consists of a common inlet and body with two outlets. This will have two bonnets, stems and wheels. Double headed hydrant valves were earlier being used where higher flows were required due to severe hazards i.e. you could connect two hoses from the double headed hydrant.
Single headed hydrant valves are standard hydrant valves with a single inlet and single outlet. A double headed hydrant post is fitted with two separate single hydrant valves, allowing two hoses to be connected from the same hydrant post.
Due to the larger body of double headed hydrant valves, there are safety concerns, and nowadays, double hydrant posts with two separate single hydrant valves are used instead.