I know this is 2 years later, however, I had the same issue, so:
The doubler cable has a female end w/2.1 mm center pin (5.5 mm outer dimension).
The HX effects is 2.5 center pin (5.5 mm outer)
Most (Voodoo/Strymnon/etc power supplies come with 2.1 mm center pin cables and they won't work
To make it work, you need a cable that will accept both 2.1 -AND- 2.5 mm center pin 5.5 mm outer. Run that cable from the doubler cable to the HX Effects
Here is an Amazon link for the cable I got
https://www.amazon.com/Power-2-1mm-2-5mm-Angled-Degree/dp/B016BJGZ2U
