Photos of the persons set-up would be helpful.
I do carry kayaks, but conventionally on a rack on my canopy. Yesterday, on I-5, I saw first hand what happens when a kayak isn't secure when being transported. Fortunately I wasn't the first vehicle behind. The kayak took out the front bumper, the hood, smashed the radiator and the windshield. Strangely the kayak, which ended up on the center line of the two lanes, didn't appear to be damaged.
Because of the length of the kayaks my first thought that just the receiver wouldn't be adequate. I'd think that there would also have to be a second stabilizing bracket also.