Plugin the Dual Extruder, flash the firmware for the M175v2, and then change the toolhead. If you change the toolhead first then the firmware is upset because the second extruder thermistor is missing and this causes the firmware update to fail.
A 100K ohm resistor across the pins for the second extruder thermistor also works (https://forum.lulzbot.com/t/dual-extruder-dummy-plug-for-second-connector/5946)