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.