Presumably this only works with two boats that are sufficiently similar for the beams to line up so there is no chance of fore-aft movement of the top boat.
I've done this with a Nacra 20 and Hobie Tiger where the spacing between the beams is different. We lined up the front beams and them made a spacer out of some 2 x 4s and hull cradles for the back. This worked for a 2,500 miles of driving too / from the Tybee 500 one year.
