I have read the H9 manual on MIDI (pages 26 and 46) and it does say that the H9 will automatically look to the usb port when it is connected.
I think the answer is "no" but my question is this: If a usb host/controller is connected to a first H9 usb port to send midi output level messages to that first H9, can the DIN port of that first H9 still be used to send preset increment midi message from that first H9 to a second H9?
I'm looking at simple (Midi Baby?) ways to use MIDI to control/change the output level of an H9 since there is no expression pedal control over the output fader. I like the Idea of using the usb port but I think if I do that, I can no longer link two H9's via DIN.
The answer is NO. If USB port is connected, that will precedes DIN and H9 will ignore DIN.
You can connect your MIDI controller to your first H9's DIN input, and connect your 1st H9's MIDI OUT to 2nd H9's MIDI IN. This should work for what you want.