It's been done before on our cars. In fact, due to the bar shape I think you just need a few fender washers between them to get the spacing needed (i.e. no need for custom spacers).
Biggest issue is that you are adding quite a bit of unsprung weight for not much gain in roll stiffness.
In rear swaybar terms on our cars, you're getting approx double the roll stiffness for double the weight (18->36 lbs). In contrast, my Hellwig rear bar (30 mm vs 26mm stock) is in the same ballpark as that as far as rear rate increase......and only weighs about 3-4 lbs more than the stock bar!
Or put another way : the monster rear HA bar on our cars is right at double stock bar weight (38 lbs). It is a whopping 440% stiffer than stock though!