1) single pipe through the tunnel is easier to fit (making room for larger DS and the possibility of a TQ arm).
2) single pipe is lighter than 2 pipes (MPG)
3) single pipe is cheaper than dual pipes
4) small gains in using a dual pipe design.
If your car "is and all out drag car" you're definitely NOT using bassani... or any other exhaust system that goes past the rear axle. Try turn downs.