Hello Aiki_007,
Thank you for posting in Q&A forum.
As I understand, the machine is in workgroup, the users are local user accounts on the machine and they sign in the machine locally, am I right?
If so, I am afraid there is no such GPO setting to sign out the two accounts automatically.
Why the two users sign out when leaves the PC station?
You can restart the PC when next day somebody wants to login on this PC.
I hope the information above is helpful.
If you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou