I have a very similar situation with controlling expression on an H9 using an ES8.
My particular issue is having one expression pedal (Dunlop Volume mini) that I want to use as a Volume pedal and expression pedal and switch between the two with a button press on the ES8.
I know I can turn the volume via exp on/off per preset, but im trying to avoid creating two patches to switch between that mode. Although when you have 99 presets, its certainly not an issue to do so.
I guess you may connect your expression pedal to your ES8, and have ES8 send two different MIDI CC messages, and map these 2 MICI CC messages to the expression and output volume functions on your H9. Then you can decide which MIDI message you want to send to your H9 by setting up your ES8 correctly and pressing a switch on it.