If you see the same thing on both displays, you should start by making sure the displays are Extended instead of Mirroring ...

Using a laptop as a monitor for another computer allows you to not only mirror your computer's screen on the laptop but also extend it, meaning you can show different content on both screens.

If you don't have a second monitor on your desktop computer, you can use your laptop's screen, and in this guide, I will explain how to complete this configuration on Windows 11.

To use your phone or tablet as an extended display, open GlideX on your PC and navigate to the Cross-device operation tab. Under the Extend option, click Search for device.

With Sidecar, you can use your iPad to mirror content to your Mac or extend it to your tablet.