Thanks PierM and Silverhead,
I understand that the HX stomp only has a single DSP, but the manual itself does state that 2 stereo paths can be used, with Parallel routing. From the HX Stomp 3.0 manual (p. 18):
Serial vs. Parallel Routing
For many guitar tones, a serial (a single, stereo path) signal flow is more than adequate. For example, the preset below includes a Pitch/Synth block (violet) > Amp block (red) > IR block (pink) > Modulation block (blue) > Delay block (green) > and Reverb block (dark orange):
For more sophisticated tones, a parallel (two stereo paths) signal flow can be created. This lets one split the signal into two stereo paths, process them separately, and mix the two paths back together.
...it's that last sentence that has me most confused: you can split the paths, but do they HAVE to be mixed back together, or can they be sent out separately?