This may not translate directly to your situation, but I think the functional principles are probably the same.
I've run a two-piece cat frame for years on my big 20' El Tigre Grande -- starting with two six foot sections. Sometimes I've bridged the two with a two foot trailer frame, which gives me 14 feet of frame. I've also used one of the six foot sections in conjunction with the solid ten foot frame from my 16' NRS cat, giving me 16' of frame on 20' of tube.
In my experience, as long as you strap the sections together well and then diagonally strap them to the outer tubes, I've never had them shift in a way that gave me concern. This includes rowing the Grand twice in that rig, and motoring it twice -- once at a steady 24k and another in fluctuating flows between 7.5k and 13k.
I figure that if it holds together under motor power in those higher volumes on the Grand, there's things I have to worry about more than the frame moving around. And I suspect that will be your experience as well using smaller components.