We have two AI's on the standard Hobie double AI trailer. It's a Hobie 14 trailer with two metal crossbeams on the frame, and the saddles just bolt onto those crossbeams. There is also a T post for carrying the masts, but that is unrelated to the saddles.
To answer your specific question, each saddle has two holes in flat base sections, and they could be bolted to anything as long as it is solid and there is about the right distance between them.
Let me know if you want me to measure mine. Here's a pic showing both our boats loaded on the trailer, plus my foldable kayak sitting atop the mast carrier.