I have 2 jacks mono connected in the out of H9 and a female stereo jack for my headphone. That’s ok for working without something else
This is a good trick.
A guitar-maker friend needed a headphone demo setup for a guitar show this weekend. He used a boost/overdrive (optional) into an H9 and then directly into headphones (with a Y cable). I think they were low impedence headphones b/c we had to boost the output level fader to +14 dB in each present. The stereo spread was very pronounced. For example, delays bounced all over the place.