Single or Dual Stacks

[SPoe]

I have a 1993 d250 with a single 6" mitre cut stack with a rain cap and the top of the stack stops at the top of the cab, the in cab drone is atrocious when going down the freeway but fine around town, since i drive 1. 5 hrs to college i was looking for some knowledgeable people about the in cab noise between a single stack and dual stacks, i kept my original 4" side exhaust but i really wanted to try dual stacks since i already have a 6" hole in the bed. so basically my questions are how much of a difference is going to duals? and how much of a difference does it make with the end of the stack ends above the cab? What i would like to do is dual 6" mitre so i can reuse my existing stack and maybe set the top of the stack between 6 - 12" above the top of the cab.