Usually when a car overheats on the highway, and not in traffic, its the radiator. I have always had a radiator shop clean out my radiators. I have never had good luck with radiator flush. Usually radiator shops charge around $50 to clean them out. They will also tell you the % of blockage you had and your flow after the cleaning. If you still decide to go with a fan infront of the radiator and one behind it, offset the fans as much as possible. Example: Front fan top left, Back fan bottom right.