Jack corrected a couple of items I was not that clear on when explaining them. So let me correct what Jack said on a few items.... Since I have made 5 of the pirogues....... One was out of Red Wood Strips ( 144 of those strips ) the rest plywood.
1... If the side panels are matched evenly then you will not have a problem , you are only making one board out of two of them. Actually you will be taking 6 boards and making three from them. Two side boards and one bottom.
2....There is no need of an offset or any form of curvature when making the pirogue. The curvature comes later when you assemble the pirogue. The end pieces and ribs determine the curvature of the sides and the rocker in the bottom. Everything is straight up to that point.
3.... I have never had a problem with the panel breaking when I turn it over , just be careful with it , place a hand on each side and just roll it over.
A butt joint the way I do it has never broken , scarf joints have. I really do not like the scarf joint , lots of work for something inferior. I need to say they my boats were made from 1/8th inch wood , this is why they weight between 32 to 40 pounds for a 16 footer and doing a scarf joint is next door to useless on wood that thin. All they do is break on me when moving them.
4.... You could use a piece of the plywood to do the butt joint in place of the fiberglass and if you do then angle the bottom of each inside one and the one across the bottom of the boat or you will be cussing a blue streak. plus using 1/4 inch wood you would have a 1/4 gap ( hole for better words) right in the middle of the boat.
When Jack built his Coho by Pygmy boats that was the way John Lockwood (Pygmy Kayaks) said to do it.
Later when I built my Coho the suggested way of doing it was with the fiberglass. That keeps the hump out of the inside center of the boat which the paddler would be sitting on part of it.
5.... I tried (on some scrap wood ) to break a butt joint with the glass and guess what , the wood outside of the joint broke , the glassed and epoxy saturated portion remained intact.
6.... Epoxy saturating the wood will/can increase the strength of the wood up to 4 times its original strength , then adding glass to it just kicks it up a few more notches.
It is your boat , do it the way you want and are comfortable with , my ideas are only suggestions , nothing more , nothing less.