When Monterey eventually supports Universal Control fully, you should be able to do smarter things with the displays and input devices of a couple of recent Macs. In the meantime, AirPlay Display could still prove useful, although I’ve seen relatively little written about it. I’ll try to explain more here, despite being unable to locate any good explanation from Apple.
AirPlay Display can be used to extend the display of a compatible Mac to include the displays of other nearby compatible Macs. It also seems to overlap with Sidecar and Universal Control, in that it can work with iPads too, although so far I’ve only had brief success when connecting my M1 Pro MacBook Pro to an older iPad Pro, which keeps disconnecting spontaneously.
To be compatible with AirPlay Display, a Mac apparently needs full compatibility with AirPlay; older models which offer reduced-quality AirPlay appear unlikely to work. Both Macs need to be connected to the same Apple ID account, and have both Wi-Fi and Bluetooth active and within range. You can also connect them using a USB cable instead of Wi-Fi, which greatly reduces latency, something only too apparent when using Wi-Fi.
When a nearby Mac is available for AirPlay Display, it will be listed from a new button Add Display in your ‘master’ Mac’s Displays pane. Simply select the secondary display there and you should immediately see mirrored displays on the two Macs. The snag with doing that from a new MacBook Pro to an iMac’s 27-inch display is that the notebook’s display is set to the same resolution as the 27-inch display by default, making it very hard to use the controls there.
To configure the two displays, click on the Display Settings… button (if you can see it!).
The upper of the two displays listed should be the ‘master’. When you switch that to its default resolution, you should find it much easier to use the controls without a magnifying glass. Here I’ve made that the Main display, as you’ll probably want it to remain.
You can either mirror to the secondary display, or use it, as I have here, as an Extended display at its own default resolution.
The final step is to drag the secondary display until it’s in the correct position relative to the master. Like any secondary display, you can then drag windows across to it, transfer the menu bar (not advisable because of the lag), and move the pointer freely from one display to the other.
Even with the lag resulting from a Wi-Fi connection, the display on a nearby Mac can provide a useful extension on which you can place relatively static images such as reference documents, although I’m not sure that I’d want to play video on it.
Returning your displays to their normal settings is simple: open the Add Display popup and deselect the secondary display there.