It took a while to feel comfortable on the highway with 2 kayaks on top and going 70 mph down the interstate. The truck rack we have holds both yaks overhead, and we strap down with more straps than we probably need.
We are using a pickup truck rack from U. S. Rack. One side has a set of Yakima HullyRollers to slide up with, and it has Yakima Mako Saddles on the other end. The other kayak is racked with a pair of Malone Sea Wings.
If everything goes right there will be a video below showing how they ride, and a look at the gear stowed all over the truck.
I had a nice youtoob video here showing how the yaks racked up on the truck, but it was dragging in too much google and doubleclick code so I removed it.