edt, I did not try all kinds of things. It took quite awhile just to make those two foils as they are welded sheet metal. I had to make a special die for my stamping machine to get the curve. I did not run any calculations on the foil surface area so you might be completely correct.
I do know I have way too much surface area and way too much cross section curve in the foils. I have a machine called a Trampofoil which is a human powered hydrofoil. You furiously jump up and down and it goes up to 14mph. It's main wing is almost 9 feet long and around 4" wide. Go figure.
Goes down the front of the slightest swell without pumping and easily slices through incoming breaking waves. I taught Laird Hamilton how to use it and he now owns two or three of them.
tks. I think I'm going to start with about 30 total inches of foil under water with two aluminum struts only an inch wide.
the lift equation is proportial to the square of velocity and linear with area so if you expect a kite to go 25 mph at cruising speed and the trampofoil at around 7 mph you'll need about 13 times less surface area, with a 9 foot x 4 inch foil that's 432 inches of wing, if you divide that by 13 you get 33 inches . . .
I think the trampofoil is also more stable than a kite foil because you dont have runaway feedback effects on a trampofoil that you do with the kite
I think to control the ride with 2 equal size foils, separated, you need a way to negate most of the lift of one of them, be it via AOA, attitude, whatever, or it would get either squirrelly or too locked into the wrong groove, probably whichever is least convenient.