: . . I'm actually building 2 x 11ft long
: kayaks, inserting a shoeplate slot into the a small cockpit and
: attaching a shoe (much like cycling shoes), then banding the two
: kayaks together and using an extended paddle for propulsion -
: voila - walk on water.

Let's go back a few steps. You probably don't want two kayak-shaped boats.

Standing with your legs wider than your shoulders gives good balance on land, but trying that on water will simply force the boats further apart, and in seconds you'll be doing the splits on the way down to the water. Then, when it is time to take your next breath, you'll wish your foot release was positioned where you could reach it.

So, let's look for a design which keeps you upright. Only the part which is underwater is going ta affect your stability, so if you are committed to having water shoes which look like kayaks just make them a bit taller and carve the part above the water to look like anything you want--- tugboats, ocean liners, swans, whales and porpoises are just a few possible designs.

Underwater you'll want something with a keel which has a good surface area to resist the side-slipping that you are going to encounter are you transfer your weight from leg to leg.

You'll also need to worry about displacement. Each float must be at least large enough to carry your entire weight. No sense capsizing at your first step.

Then you want to have the hull shape help your propulsion. It should resist moving backwards when submerged more (as you put your weight on it) but be easy to move forward when less weight is on it.
It sounds like you'll need a seriously asymmetrical design, and make mirror images--- one for each foot.

Have fun with the design. :)

I'd try to keep the centerlines of my feet no more than 9 or 10 inches apart. That means some very narrow pontoons. To get the displacement you'll need the hull will need to be very narrow, long and deep. At a point where half your body weight is displaced, the hull shape will need to flare wide so you can displace your total weight on a single hull while you move the other foot forward. Very unlike the hull lines of a kayak. If you look at the lines of an aircraft carrier you might be heading in the right direction. The conning tower is way to the side and the deck flares away from the hull. Now picture a similar shape, but move the keel so it is directly under the conning tower. Ake two of these models--one with the conning towre on the left and one with it on the right-- and put your feet where the conning towers go.

That gives the cross section. From the top view I think you might want to consider a Swede-shape design. That would put your center of gravitywell behind the centerline of the boat. With the bow more bouyant than the stern it might be easier to plant your weight on one sidewhich would have more drag and resist moving backward while you slid the other foot forward.
Using a strap between the two hulls would keep you from doing the splits, but it would also limit the length of stride, and perhaps the forward glide. A strap would also limit your ability to shift weight from leg to leg.

Just some thoughts on this. Hope they help.

PGJ