I should also mention that in the document I linked to we recommended a few expression pedals that we know work when used simulataneously with an aux switch. When we were testing this functionality, we found quite a few expression pedals that wouldn't work in this configuration. So, if you would like to use an expression pedal together with an aux switch it would be advisable to use one of the combinations that we suggested in that document. We had a customer try this with an expression pedal we hadn't recommended a few weeks ago, and he ended up having to take his expression pedal apart and resoldering it to get it working. He was able to get it working, but of course a lot of people wouldn't be comfortable with taking an expression pedal apart and trying to resolder it… And of course we can't really provide support for someone who needs to do that either…
