Though I've connected both extruders and double checked the wiring but I can only get one extruder to display on the touchscreen. Doesn't matter which of the two plug positions on the touchscreen or which extruder is plugged in. Do I need to reinstall the touchscreen firmware? How would I do that?
Seems you loaded the single extruded firmware on the touchscreen.
in DSE setup guide, step 6 explains how to load firmware on the touchscreen. It also has the right links to download the firmware