There are really too many possible combinations of pedals, switches and cables for us to deal with each one individually. So, we will test a few combinations of cheap and widely available parts, and put out the information. While we are doing this, here are a few tips:
1) You must use H9 software V4.8 or above. (Didn't work properly before).
2) You must use a Y-cable with a stereo (TRS) plug on one end (goes to H9) and two mono plugs (TS) on the other end. The TS connecting to the tip of the TRS goes to the expression pedal. If you use TRS to TRS, the pedal and switch will interact.
3) You must use an expression pedal rather than a volume pedal. If you use the second, you will find that it goes 0-100-0 as you swing the pedal. You can turn a volume pedal into an expression pedal by cutting the wire that goes from the pot to the ring connector on the jack.
4) You should use a momentary (non-latching footswitch)
The above pretty much covers it, but more later ….