New Ion XE 20 only one monitor is touch screen enabled. Is this a setting I've missed or a problem with the screen?

you are saying that you have 2 screens that are touch screens, and only one is working?

2 options here:

1. go in to the shell to monitor arrangement and us "configure Touchscreens"
2. change the USB cable from one Screen to the other and check if it is working. I recently run into an issue where one USB cable refused to work with one of my screens, but worked well with the other. In my case, i will buy an other cable. But i also could pair Cable 1 with Screen 1 and Cable 2 with Screen 2 as a fixed pair (not working on rental : )
3. make sure all Cables are connected properly.