I'm presuming that your cut in the middle is a metaphorical cut and that you would lengthen the plans by six feet and build a second kayak. I can think of two downsides to cutting an actual boat to lengthen it. One, it is very hard to keep fair lines, and two, you concentrate woodstrip joints in two narrow areas at either end of the new section. ON the other hand, stretching the plans is entirely doable.

Mike Scarborough
Friendship Kayak