Before macOS 12.3 and iPadOS 15.4 were released to the public, many beta testers who tried Universal Control were unable to get it working if they were already using an iPad in Sidecar mode, but it turns out that in the final release you can actually enable both Universal Control and Sidecar simultaneously on different devices, provided your setup allows for this.
In the final version of macOS 12.3, the “Add Display” options in the Displays preference pane distinguishes between devices available to connect via Universal Control and devices available to be used as extended displays, including iPads via Sidecar.
This means that you can now, for example, connect your Mac to one iPad via Sidecar for some extra macOS screen space, and on the other side of your Mac you could control a second iPad or Mac using your main Mac’s keyboard and trackpad. You can even enable Sidecar and Universal Control on the same iPad, allowing you to swipe up on the Sidecar “app” to reveal your iPad’s Home Screen with active mouse/trackpad support and Sidecar minimized in the Dock.
Read more at MacRumors.com