I've set up a couple of HDMI displays here in my lab and they appear to be working just fine, so need more info from you. Please right-click your desktop and select Graphics Properties. Click on Multiple Displays on the left. Then on the right-hand side of the window it should allow you to change the Operating Mode. When setting mine to Clone Display, both monitors work as expected in Windows.
Let me know what you find in the properties. Also, what are the makes/models of your two monitors?