Well did some digging here is another that shows how they work. Just clamps around the roof rails.
Kinda nice that they are so universal. They will fit pretty much anything with rails. Its nice the J racks hold them high. The rear hatch opens fully and around town you don't even notice them up there. If your short you might have to bring a step stool as I'm 6.3 and its right at the limit of reach for me I am lifted though. Comes with all the straps and stuff you need. I don't use the bow and stern straps just the ones on the racks. I took them offroad up steep hills and battered them with trees its really solid.
If you don't want to go the j rack route you could go with hully rollers. Pretty much the kit contains wheels which mount to the rear cross rail. A rubber hull block attaches to the front rail. This allows you to push your kayak on to the roof from the back which is easier then lifting the kayak over the truck. Not positive you could fit two side by side though. I know it would work if you did one j rack and one roller.