Sounds like your biggest worry is driving 50. Where in the U.S. is there 55mph highway that people aren't driving 70+? Driving 50 sounds downright scarey.
I drove from Maryland to Arkansas and back with two boats exactly as you describe. I went about 65mph. So long as the racks don't rip off the vehicle and they are tied on solid, you should be good to go. Your gas mileage will be among the worst you ever got, but that's nothing to do with safety.
I don't have gunwale/load stops on my rack, and I worried about side-to-side movement in strong cross winds and behind 18-wheelers. There was more of that than I cared for. If you don't have stops, use those tie-down skills to make sure the boats don't migrate to the side.
I think it would also have helped my situation had I positioned the boats center-racks. I tie them on with more sticking over the hood, the center of the boats are just behind the front rack bar. In side winds, the front of the boat wants to twist downwind. If they were more centrally balanced over the bars than the truck, it might have been better.
Wall Mount Boat Racks
Electric Kayak Motor
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