Joe,
I stretched a GE to 20 feet and she has been fantastic. Mines a single but I'm guessing you could make yours a two holer fairly easily.

I did a lot of nip and tuck on my GEE plan tweaking for slightly differnt performance characteristics. Besides tweaking is basic to my nature. I added a inch at each station for a smooth transition and then 9 inches bow and stern. I wanted additional rise in the bow and stern.

In your situation, I might suggest leaving the bow and stern alone and instead add your inches nearer the middle stations to get a longer swatch of boat where folks sit.

You'd have to play around a bit to figure out what looks/feels best, but if you added two inches to your form placement stations in the middle then dropped to 1 inch additions to your form placements then down to the original plan of set s nearer the bow and stern you might end up with a 20 to 22 footer that might transition form to form easily give ou the most additional lenght where you need it and be a slick project.

Remember that when you add to a boat for a double you need to add more up front deck for the bow rider. You want a bit of deck to keep them dryer. And they'll need leg room. Unless your front rider is canine. (not all that uncommon)

Also think about putting your big storage hatch between the two cockpits.

Good luck, sounds like a neat bit of nip and tuck adds and pads. Should be fun

!RUSS