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.