I can be completely wrong on this, and I don’t want to burden anyone with my understanding, or lack thereof, of the benefits of biscuits.
But here goes;
In my view, Biscuits don’t add significant strength to a joint; and the doubling-up attempts to compensate for this. I can’t speak to Bill’s reasoning; however, I’ve always thought of the biscuit as a helper, not a solution. When you can double-up, you’re adding a little more help.
The whole biscuit premise relies on pulling moisture from the glue, to expand, and fill the void formed by the cutter. So the hydraulic pressure of the expansion is directly related to the amount of moisture available to the biscuit, the swelling-factor of Beech, and the size of the void it has to fill. Beech expands a given amount, and creates a given amount of hydraulic pressure. I’m sure there’s more to it, but this is what I think I know.
Again, I can be way off-base.
MJCD