That double sway bar thing is a first for me too. I noticed a lot more stiffness in the rear-end of my road course 02 GT when I put in the rear shock tower brace. Don't know how well two sway bars might do though.
I confess, adding a second bar to a Stang - it's the first I've seen. I suspect the Steeda bar was developed for cars that had no rear bar originally. But, maybe not...
Michael,
If you read Steeda's website in that link, they state that it was designed to compliment the stock bar...so it was designed for cars with the bar already on them.
From Steeda's Website
Steeda's Adjustable Rear Swaybar works with your stock rear swaybar to dramatically improve handling.
Well, there you have it....it would appear it's there just as it's supposed to be. Although one would wonder why they just didn't produce a stock bar in a larger diameter to save all the extra weight and trouble...