http://www.jacksonkayak.com/kayaks/product.cfm?product=dynamicduo
Could be great fun with the right crew, or highly entertaining for the spectators with the wrong one!
Neat
Looks very similar to the Topo-Duo to me.
http://www.eskimo-kajaks.de/english/02_topoduo.html
Nice to have options however.
You know about the composite k-2
available from PS Composites. While one is certain to break a composite boat now and then, it will hold form better than a plastic k-2, and probably perform better. In order to stiffen a poly k-2 enough for good performance (and safety from entrapment), one has to put in enough bulkheads and foam to make the boat even heavier.