It itself is not FOSS, but Network Chuck just had a decent video on setting up ThinLinc for his editing team. I believe it is free for ten or fewer users. There is an admin function that allows you to observe what another user is doing (session shadowing).
https://m.youtube.com/watch?v=qdo5lMR1lX4
In general, for native FOSS, I’m pretty sure you can specify the screen port in VNC, and connect multiple users to the same port, but you’d have to double check the configuration file. Can likely also be done with XRDP.
The FBI and prosecution should do their fucking job and build a case, instead of relying on illegal search and seizure.