This definitely is Multiplicity, but it has nothing to do with the PCs being idle. I will be actively using them and have my main PC just stop accepting keyboard/mouse input and unable to do anything.
I have a shortcut on my other PCs that I control with their own keyboards that basically makes a remote windows services call to the main PC to restart Multiplicity services, which restores the main computer's keyboard/mouse.
Prereq: You must enable other PCs to manage your windows services with your user that has admin rights on the target PC. You can enable this on a PC with an elevated Command Prompt. You may need to reboot after adding this command:
> reg add HKLM\Software\Microsoft\windows\CurrentVersion\Policies\system /v LocalAccountTokenFilterPolicy /t REG_DWORD /d 1 /f
Prereq: You must install PSTools and have the exe files extracted somewhere in your PATH:
> psservice64 \\192.168.1.100 -nobanner -u username -p password stop multiplicity /acceptEULA
> TIMEOUT /t 5 /nobreak
> psservice64 \\192.168.1.100 -nobanner -u username -p password start multiplicity /acceptEULA
Replace 192.168.1.100 with the computer's IP address that needs Multiplicity services restarted. Replace username with your username. If using a local user account, use the name you see at the login screen. If you've enabled Microsoft account login, you can't use your name on the login screen, but must use your email address. Additionally, if you're enabled PIN login, with a Microsoft account, you use your 4 digit PIN to login as the password. If a local user, use the regular password.
NOTE: This does store your password in plain text in the script. You can do something else, such as store the password in the registry and read that at run time, which isn't necessarily more secure, just more obscure.
This works every time to fix Multiplicity on the PC that stops accepting all keyboard/mouse input.
Hope this helps!