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.