Paul Stomski did something like this. he wanted a cross between an open sit-on-top and a closed kayak, and he also wanted an optional triple. ( yes, these were weird combinations!)

He made two "hatches" big enough to be cockpits, fore and aft of the center cockpit. With the hatch covers removed, he could pop in a seat and backrest, and a child can fit in each spot.

The center area he made a removeable deck section, going down to the sheer. With this section removed he could fill the center with foam panels. He was makeing a seat to fit in there so he could sit a bit high, on the foam, and be exposed like on a sit-on-top. With the foam pulled out he could replace the center section and have a conventional cockpit and coaming.

Bulkheads at both ends of the cockpit area provided a fastening site for the center section of deck, and prevented water from goign from compartment to compartment.

Should be some pictures posted. check the old archives for postings by Paul Stomski.

hope this helps

PGJ