I have a requirement for three users, myself and my two grandaughters. I've found a way to create and restore the Ubermix image manually, but it would certainly be a great help if it were included in the normal scripts.
This is how I did it :
Create a standard install and re-boot into full customisation so you are editing the default install.
Edit /etc/lightdm/lightdm.conf and comment out the two 'autologin...' lines, this puts up the login menu.
Now create the additional users, I created two 'standard' users, set up and test in the normal Ubuntu way.
Update the USB memory stick Image, then do a Simple Update, which creates the USB IMAGE partition.
Re-boot into full customisation, change to root user and compress the two additional users 'home' directories
Copy the two compressed files to the USB IMAGE partition.
The USB memory stick can now be used to install another system with the additional step of booting into full customisation mode and uncompressing the two additional users 'home' directories from the USB memory stick..
This works for me and would be a useful addition to the standard system scripts if it could be implemented.