We're talking about a sudden unexpected flush. If I had a bird pointed and held I would cock both hammers and shoot right then left. I didn't say it was for every situation. I do think in driven shooting the left barrel was often used first as birds came into range and then the right as the birds were closer. No set rules just adapt to the situation.

If we all went by W.W. Greener's recommendation, we would just walk around with them cocked at all times!