Crossed or not at hwy speeds it isn't going to matter as the chains will break and the trailer will do what it wants. Now before all you know it alls start in I have been a part of 3 camp trailer roll overs and a flatbed trailer hauling 2 atvs coming lose and all 4 trailers broke their safety chains. The 3 camp trailers ended up on their sides and the flatbed trailer crossed the road and stopped in a pile of dirt. From the reports of the drivers of the camp trailers they all said they figured the trailer was going to flip the truck or push them off the road before it broke loose and when the trailer broke loose they were able to regain control of the truck and stop safely. Now at low speeds the crossed chains or chains that are not to long will support the trailer off the pavement so when you are trying to stop the trailer will smash into the back of the tow rig.
Heres a good one, had a guy tow a trailer 400 miles in one day, as he was pulling into a motel parking lot he hit the curb with the right trailer tires which pulled his hitch out of the receiver. Seems he towed 400 miles without a hitch pin installed.