Mike,
That's just it. The cove has a hard time holding up during construction. Al lthe on and off. Testing the mate etc. planing off the cove is a more forgiving method. Its easier to recover if there are small gaps. Also there is a fail safe in flattening the sheers. When you go to mate both sections if they don't line up perfectly its easier to just plane down the high spots until everything is perfect.

Using the double cove method. You will have to be pretty close on to perfect both horizontally and vertically. If your off by a little bit your going to have to rebuild work the amount your off and at the same time rebuild the cove.

You can do mate the sheers using the double cove method, but only if your certian your going to come out perfect, and if your sure you can protect the routed shape for the life of your project.

My suggestion would be, if its your first boat, Or even if its just the first boat off of this form. I'd suggest the planed method. Shake the bugs out of the form on a couple of boats then if you stil lwant to try double cove try it out on a proven form.

Good luck enjoy
!RUSS