When both of your monitors are showing the same thing, this usually means they are in mirror or duplicate mode. They can be separated into independent displays in your system settings by doing the following.

On Windows, right-click your desktop and then click Display Settings. Click Identity to discern which monitor is which, then click on the corresponding number box to select the display. Finally, click the drop-down menu labeled "Duplicate these displays" and change it to "Extend desktop to these displays".

On MacOS, navigate to System Settings > Displays. From here click Arrange/Arrangement and then uncheck Mirror Displays.