I know the 9 meter Wosling kite is frames in close to 3/4" and the 12 meter one is framed in 1". I haven't asked Bernd in a few weeks about my 5 meter kite. But hopefully will get that heading my way soon. 15-18 foot dual, I would say that Icarex should be fine, and would agree that the frame, stitching, bridal, and line set will be more of a concern. Thor's Hammer is 12mm and 10mm framing. The problem is finding the fittings. If yo find a source, please let me know! I would also think that if you framed in 12mm it would be strong enough.
Nator