Boiling Malc's explanation down, the mechanical properties of the sway bar are the sum of all the pieces, and it effects both sides equally as the roll stiffness. The roll stiffness doesn't change due to which side of the car you're measuring. If the blade doesn't have the range of adjustability then it'll do you good to double up (as pointed out earlier). Either way, the effective roll stiffness is the sum of the arms, links, bushings, and crossbar.
Having 2 blades isn't a bad thing, but it's not necessary.
Chris