[CentOS] Mirroring two screens for all users?

We would like to have one workstation (CentOS7 with Gnome3) set up that 
has mirrored/cloned screens for all users that may login

The workstation is using an Nvidia (proprietary) driver for the graphics 
card. We have set up xorg to use cloned screens, but gdm/Gnome3 appears 
to ignore these setting and only use what is set in ~/.config/monitors.xml

We've set up /var/lib/gdm/.config/monitors.xml with mirrored/cloned 
settings for the login screen, but when a user logs in, the display 
switches to spanning both screens

Is there any way we can force all user logins to use cloned displays on 
this one workstation (but not affect any settings they may use on other 
workstations - all users has NFS mounted home directories) ?

Thanks

James Pearson