RE: Double fins?
I agree with Roger.
Twin fins are better under limited circumstances of sub-optimal fin choice in the single fin board because of wind / depth conditions. In that circumstance, they are demonstrably superior.
I would point out that there are only a very few shallow / wide fins for single fin boards, and I'm not certain how they would compare.
Certainly, experience shows that the fastest configuration is the minimal fin that allows planing, and the highest efficiency is a high aspect ratio pointer. So when water depth is not limiting, I can't imagine a single fin not being superior.
Just ask yourself....when's the last time Boeing made a biplane?