In theory it will hold it. The bottom of the hulls will probably sit on the rack itself and not even touch the crossbars (which are the weak point in an already weak system). Long ratchet straps over the boat and under the main rack will probably work, but maybe add some sort of cushion between them. As itr1275 mentioned, the front must be tied down to the front of the 4Runner. I worked in the windsurfing industry for years, and the number of boards and racks that peeled off the top of cars because they weren't tied down to the front of the cars was frightening You are liable if your load comes undone no matter what causes the failure.