How do I set up multi seat?

The Ubuntu Wiki has some instructions for 10.04 on this. Also mdm is mentioned sometimes, and maybe you can follow this 10.10 tutorial.

As far as I can tell, there's no (free) GUI-which-works-out-of-the-box solution for this. I guess what you have to do is to adapt your XOrg configuration file and tell it which monitor-keyboard-mouse combinations go together.

The best resource I can point you to is this AskSlashdot thread. You'll find plenty of links there. But beware: it looks like you should have some linux experience, especially with editing config files etc.

Ubuntu 13.04 (Raring) and 13.10 (Saucy):

Ubuntu 13.10 (Saucy) switched to logind, which has much better multiseat support than ConsoleKit (the seat management software used in 13.04 (Raring) and older). Much of the work that went into switching to logind is actually present in Raring, so with some effort it's possible to use logind with Raring.

Despite switching to logind, certain key packages (notably lightdm and xserver-xorg-core) still have some issues with multiseat support. The ubuntu-multiseat team is actively working on uploading fixes to these issues to the ubuntu-multiseat PPA.

To set up multiseat on 13.04 (Raring) or 13.10 (Saucy), you can follow the instructions on the ubuntu-multiseat PPA page. The following is a summary of those instructions:

1. Add some udev rules to assign devices (keyboards, mice, video cards, etc.) to seats. Details can be found on the PPA page and the systemd multiseat page.
2. Edit /etc/lightdm/lightdm.conf and tell LightDM about your seats.
3. Update to the packages in the ubuntu-multiseat PPA.

Setting up multiseat correctly is currently very hard; hopefully this will get easier in future versions of Ubuntu.