Once the mechanical installation is complete, You need to enable the second extruder in the Duet configuration.

Open the Duet Web Control. Go to Settings, then go to System Editor. Click on config.g to edit it. (Pic 1)

Scroll down to the line ";Dual Extrusion Code" and the 2 lines below that need to be un-commented. Remove the semi colon from the beginning of the M563 and G10 lines.

Click Save Changes. You should get a popup asking to restart the web control to enable the changes. Click ok and you should now see two hotends enabled on at the top left of the web control when it re connects.