I think your talking about 'double planked' not double hull..

To make any repairs worth while the cost and effort, you should stay with
the traditional or prescribed method of replacing the planks. Ply wood
planking has none of the strength and longevity that are needed in any boat
planking.. Leave the plywood for plywood boats where the minimum edge grain
is exposed (and then it should be glassed).

First of all the grain of the planking should run parallel with the plank
edges, so a 7ply plywood plank would have only 5 plys parallel and 3 plys
cross grain. Hence you have reduced the strength to ~ 4/7th what it should
be..

Also, all of those end grains in the seam would tend to wick moisture into
the plys. Even a double planked hull will have movement in the seems and
eventually the seam filler will let some moisture in.


--
My opinion and experience. FWIW

Steve
s/v Good Intentions