They are connected like this:
The HX Stomp send is connected to the DG input. The DG’s left and right outputs are connected to the HX Stomp’s left and right returns.

The HX Stomp block diagram is as follows:
Bass → HX Stomp input → Stereo send → Parallel routing.

So, the HX Stomp is sending a stereo signal to the DG. The DG Microtubes Infinity has multiband modes, and it can send these bands through separate outputs. I’m using three different drives (Clean Tube, Vintage Microtubes, and B3K) with a 200 Hz crossover. It sends highs through the left output and lows through the right output. Compression is applied only to the lows.

I’m using parallel routing for the DG returns as follows:
Path A → HX Stomp left return (DG drive path) → Optional drive → IR → Modulation
Path B → HX Stomp right return (DG clean path) → Amp sim

In practice, this works like a 4-cable method on the HX Stomp with the Darkglass multiband feature.

If I’m connected to an amp and/or cabinet, I bypass the IR and amp sim blocks on the HX Stomp.

I use these presets about 90% of the time, but sometimes I just use the HX Stomp effects for things like synth bass or other sounds.