The so-called double brace is likely more effective than the stock one but the upper bar still attaches and transmits loads only thru stock sheet metal tabs & bolts. All are hand-made and the engineering problem I see is, the lower brackets attach to a compound-curved surface on the face of sheet metal inner rear fenders. Because the brackets only attach to sheet metal, at least 1/4" thick steel is normally used for stiffness. Forming such thick steel to a compound curve so it can be properly welded or brazed to the fender panels is a lot of metal pounding, and preload adjustments with 4 heim joints are more complicated.
Thee was a third type produced in very small numbers for awhile, consisting of two 3" or 4" thick shaped triangular carbon-fiber parts that completely filled the area between the two inner fender panels, from the upper tabs all the way down to the subframe rails. There was a threaded clevis between the two shaped parts. Rather elaborate and showy- I don't remember now who built them- but no tech article was done and I never heard anything about how well it worked. There are a few photos in the POCA Archives somewhere.