I decided to make a drawing of how this would work with the HV-Maxx, including the internal connections that the HV-Maxx uses. I am doing this because I want to see if anybody can locate a short in the system (not saying there is one, but this will help see if there is one).
EDIT:
It works the same way with the EVX.
If someone wanted to overcome this issue, he/she could simply disable the BEC circuit, put a jumper on the non-BEC input, use a UBEC/receiver pack, and put the two main packs in series. Then, the ESC would work just like any other single-battery ESC out there.
Here's a pic of what I mean: