Sami Fathi, MacRumors:

While the feature was previewed at WWDC in June, it’s yet to make an official appearance in any developer beta of ‌macOS Monterey‌ or ‌iPadOS 15‌, leading to speculation that the feature may be delayed to a future update to the operating systems. However, in the latest ‌macOS Monterey‌ beta released on August 11, Universal Control can be enabled and used between two Macs.

And this, in bold:

Universal Control is not yet officially enabled in the latest macOS beta, and the steps needed to enable it are complex. We don’t advise users to attempt to enable it because it may damage crucial system files and their machines.

If that doesn’t put you off, here’s the magic. Looks like it’s macOS only, for the moment. Backup your Mac before you dig in.