That’s what I’m wondering about. I assumed that standard CV power supplies (like in USB charging adapters) would be used. But perhaps the key is using CC/CV power supplies like your HLG’s with their built in CC current limiting.
Diode the outputs, to keep one from possibly feeding into the other since never exactly same voltages, and then let each just current limit to the load.
I don’t think any USB charging adapters use CC/CV supplies though. Just protected CV supplies. CC/CV just isn’t needed for their intended uses.
I think CC/CV supplies with PD interfaces would need to be designed. But then why go USB PD at all. You could just plug a CC/CV supply at the right voltage into the board. 
Yea, it needs to work with stock USB charging adapters.