I don't think this is going to work out. See this manual (https://static1.squarespace.com/static/539f0880e4b0e0318ea3192f/t/5940029fb8a79bde2d979ab5/1497367242457/OX9-Manual-2017.pdf).
It says "When using an expression pedal, the H9 only allows you to have one additional auxiliary footswitch (the left one)"
