Simultaneously Use Sidecar & Universal Control In macOS 12.3
Previously, some testers failed to enable Universal Control when they used iPads in Sidecar mode. However, it seems that the final releases of macOS 12.3 and iPADOS 15.4 will put an end to this situation and allow you to simultaneously apply Universal Control and Sidecar modes if Mac's configurations settings enable you to do it.
Basically, Universal Control lets you use your cursor and keyboard on multiple Macs and iPads while Sidecar allows you to mirror the content from your Mac and display it on your iPad.
From now on, you will be able to make use of the "Add Display" feature and choose devices which you would like to connect through Universal Control and which devices you would like to access as extended displays via Sidecar.
In other words, you'll be able to apply Sidecar mode to connect your computer to an iPad and thus get some more screen space. At the same time, you will be able to use your Mac trackpad and keyboard to take full control over your other devices.
