Yes, it is possible to control multiple locations with the same user in the Mill app.
You do not need to create a new user if you have multiple locations with Mill WiFi heaters, for example, your primary home and secondary home / cabin. You control all your homes with the same user.
You can easily switch between your locations at the top of the home screen in the Mill app (see below):
Follow the instructions in the tutorial video below to switch between houses: