I have seen that some people have been looking for a solution to this problem. I found an article that shows you how to do this. Simply follow the steps and it will work. Basically, you have to "fool" windows into thinking there is a secondary monitor plugged in. This is done by running some commands in CMD. After that you can simply use Google Chrome to cast the secondary display.
Timoneasthouse
I have seen that some people have been looking for a solution to this problem. I found an article that shows you how to do this. Simply follow the steps and it will work. Basically, you have to "fool" windows into thinking there is a secondary monitor plugged in. This is done by running some commands in CMD. After that you can simply use Google Chrome to cast the secondary display.
https://www.amyuni.com/forum/viewtopic.php?t=3030
Enjoy!