All right, I took some time to dig through the rule books. I could not find anything about double-boots-with-power-half being illegal, per se. That is, no penalty if attempted.
However, I believe the interpretation is a potentially dangerous call. Going back as far as 2011 (when wrestling was invented), the rule books through the 2016-17 season illustrate the maneuver as potentially dangerous in Figure 88 (accompanied by Fig. 87 showing single-boot-power-half). Interestingly, they dropped that Figure 88 in the latest rule book, now showing only the single-boot-power-half (as Figure 90).
At some point, I swear the double-boots-power-half had an interpretation change such that it was an automatic stoppage for PD. The fact we never see it attempted anymore, and that they don't even illustrate it in the rule book anymore, is telling (IMO). And I distinctly remember being pissed when the new emphasis came out.
EDIT: In the 2009-10/2010-11 rule book, the pertinent figures are No. 89 and 90. There, the caption reads, "The defensive wrestler is unable to move his hip (88) or hips (89) to relieve the pressure on his shoulder." I think the (88) and (89) are typos, meant to be (89) and (90), respectively.
The same 2 figures are in the 2007-08/2008-09 rule book as No. 86 and 87, but their caption reads the same as for 2011-2017.