I know it’s possible since I saw tabs use 2 channels at least before, and I bought Hooktheory, so it shouldn’t be a problem, yet there is no option for it.
The 2-voice thing is an oddity involving how TheoryTab converts Hookpad tabs.
If you submitted a TheoryTab via the old Flash-based version of Hookpad, TheoryTab would keep the voices intact. In the HTML5 version, TheoryTab hides and mutes the additional voices. When you copy the TheoryTab into Hookpad, the voices are still there.