if your scale can handle the total weight of the boat, "tare" (zero) the scale with the beach wheels on it, and then go roll your boat (balanced on the wheels) on the scale.
This could work with two scales, but I want to see how you balance everything on one.
Also, you'd have to be damn precise about finding the balance point so you could let go of the boat long enough to read the scales (holding on to the boat to help balance it is going to throw off the measurement).
