Decoupling Frames...Uh, just to be sure...

Greetings! Below are some pictures I took of a "closet" that will be constructed for my breaker panel and a picture of my West wall outer & inner frames. The outer frame (and the back side of my house) is towards a 2 lane road with a fair amount of traffic. Trucks do pass on this road, and when they "gear-down" you can hear the sound down in my basement. I am doing the 2-layer 5/8" drywall "beef thing" between each OUTER frame wall cavity which includes green glue between the "beef" drywall panels in the OUTER FRAME wall cavities along with caulking. I am also using green glue between the INNER frame sheets of 5/8" drywall running across the entire wall. In making the closet for the breaker panels, I was going to place Homasote around and under the front cover of the breaker panels and enclose the inside of the "closet" with Homasote to make the "closet" completely closed in. The problem is that the Homasote on the inner frame will be touching the Homasote on the outer frame (only in the manner of connecting drywall (Homasote) corners in "regular" corner construction by leaving a gap between the drywall (Homasote) corners, using backer rod and caulking). This particular inner and outer frame is more critical than any other because of the outside noises (trucks etc. etc.). I'm wondering if I should simply put a "door" to the breaker "closet" on the inner frame and not build a complete enclosure under and around the front cover of the breaker panel. Nothing would touch from one frame to another. This would mean once the breaker panel "closet" door is opened, you would be able to see between the inner and outer frames (You wouldn't see a LOT...I will only have about a 2" gap between frames). By reading this post... viewtopic.php?f=2&t=12687 I was thinking the Homasote would decouple the 2 frames, but I need to be sure before I construct the closet, install & connect the Homasote panels and make a BIG mistake in transmitting those mid to low frequency (?) sounds/vibrations frome one frame to the other. I called Homasote and talked to their tech, and my impression was that the Homasote "reduces" sound transmission when using it properly with drywall. (I DEFINATELY plan to email Homasote to get written clerification on this, heck, maybe I've got this ALL wrong, but between John's comments and the tech's, this is too critical an issue to not be sure of...at least for me...) Anyway, I'd sure be interested in finding out ya'll's thoughts.......John...you there????? :)