It may be the case that Windows is seeing your UM2 as a 1 channel interface only. To fix this, open the Sound Control Panel and open the recording tab as below:
You need to select the UM2 Audio Codec and click Properties.
Then go to the advanced tab and select a 2 channel option from the drop down at the sample rate you want to use and click Apply.
You will now be able to record both inputs.