The Xenyx mixers feautre a stereo USB bus or recording the Master Left and Right signal to computer software.

It may be the case that Windows is seeing your Xenyx 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 Xenyx 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 left and right as stereo signal.