Hi,
That's because it depends on the version of Windows you use.
Either click on the disabled monitor and change in the drop down to remove "disconnect that display".
Or find the monitor in the drop down and change its parameters.
Here is a tutorial for Windows 10:
http://www.tenforums.com/tutorials/2...dows-10-a.html
Should you use another version, you would find similar tutorials on how to use this Windows feature.
Kind regards,
Alban