As reported here:
Updating to Mesa 10.3 causes multiseat to stop working with Arch Linux and lightdm. The second seat becomes a black screen on boot. Is not specific to card manufacturer - problem exists with Nvidia, AMD and Intel configurations.
Turns out that it's a LightDM issue. Problem is solved by reverting to lightdm-1:1.10.1-3-x86_64
Discussion on the solution for LightDM are here: