2 left heads doesn’t make sense to me. From factory, temp gauge sending unit is on driver’s side towards the front of the truck, or left side of head. Passenger side has temp light sending unit towards the rear of the truck, or left side of head once again. So does that mean factory used 2 right side heads on our trucks?
I’m guessing you have heads from a different application then? Running the numbers on the heads will tell you for sure.