I'm testing Universal Control with an iMac 24" 2021 + an iPad Pro 12.9" 2021 + a MBP 15" 2018.
In this beta 1, you can only link one device at a time. The iMac is in the middle. I can only link with the iPad or the MBP. Not both.
I hope it changes as we get closer to the GA.
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