Two ways to achieve this:
First way: Plug your expression pedal to your ES5, and map it to a MIDI CC message on your ES5, so it can send this CC to two H9s, and map this CC to control volumes on both H9s.
Second way: Plug your expression pedal to the first H9:
- Set first H9's midi output to MERGE (so it can thru and transmit its own MIDI messages).
- Turn on the first H9's MIDI CC XMT
- Set the first H9's MIDI XMT CC mapping for the expression pedal in the XMT CC menu (by default it is CC 11)
- Map this CC message to Expression pedal on the second H9
- Now you may use your expression pedal on both H9s. If you'd like to use is to control the input or output volume, you may set it on a preset by preset basis. You may do it in H9 Control by clicking "More" and "EXP controls input volume" or "EXP controls output volume"