systemd only allows one graphical session per user. To allow an arbitrary number of greeters to be displayed on an arbitrary number of seats, systemd recommends greeters implement the systemd-userdb Varlink API to dynamically allocate greeter uids and gids in the 60578-60705 range (mentioned here).
I'm not sure how atrium handles this requirement, but gdm used to need to pre-allocate uids for this (gdm-greeter, gdm-greeter-2, etc.), but added the gdm-dynamic-user-store (source) to implement this dynamic allocation and integrate with systemd-userdb.
systemd only allows one graphical session per user. To allow an arbitrary number of greeters to be displayed on an arbitrary number of seats, systemd recommends greeters implement the systemd-userdb Varlink API to dynamically allocate greeter uids and gids in the 60578-60705 range (mentioned here).
I'm not sure how atrium handles this requirement, but gdm used to need to pre-allocate uids for this (
gdm-greeter,gdm-greeter-2, etc.), but added thegdm-dynamic-user-store(source) to implement this dynamic allocation and integrate with systemd-userdb.