When you turn on the taillights, does just ONE of the two bulbs light up? When you step on the brakes, do BOTH of the bulbs light up?

Maybe there are supposed to be two dual filament bulbs in each fixture that each light up for taillights and all 4 filaments light up for brakes?

1950 Chevrolet 3100 (Ol' Roy)

1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)