[tubman]

They are both "primary barrels and are designed to open at the same time. Your car has what is called a "dual-plane" manifold where each throttle "barrel" feeds 4 cylinders. The manifold is designed to distribute the fuel mixture to each cylinder in the most efficient way determined by the engine's firing order. Take a good look at your intake manifold and you should be able to follow the eight separate paths the fuel takes.

Progressive two barrels are a much later development than what is on your Ford. I think they were developed by Weber. They are generally used on 4 cylinder engines and are rarely (if ever) seen on eight cylinder engines.