Clutching and double clutching was optional and dependent on the situation. When shifting in soft footing off-road conditions it was advisable to be ready with the clutch if needed, but once on firm roads not necessary. With few exceptions that came with experience, the one rule to follow when learning to drive a main and brownie set up was to never get both boxes in neutral at the same time as you would lose the ability to match the engine speed to the rear ends.
Now, I have a question relating to the video that you posted for the other fellas that have been around a while.
I have two trucks that have Cats with retarders, and each of them have the second shorter lever attached to the right side of the steering column with which to activate the retarder. The truck in the video also has two levers, but is obviously Detroit powered. Is it likely that this particular 359 was repowered from a Cat to a Detroit, or is there another function for that second lever that I am not aware of?