Benn
I've seen alot of cages for oval lap race cars, alot of them have a x in the roof.
If you are referring to the Rookie rods / Speedworth / Autograss Class 2 / Class 6, then they have extra roof bars as they are liable to be crashed into when led on their side, Mine is just a track car, Should the worst happen it wont be anywhere near as bad as a full throttle impact.
Take a look at the Leading manufacturers designs, you wont find twin roof bars on any of them unless built from T45.