The multi-monitor situation
Please upgrade your nVidia driver to version 302.17 or newer and i3 will just work. This document is kept around for historic reasons only.
1. The quick fix
If you are using the nVidia binary graphics driver (also known as blob) before version 302.17, you need to use the --force-xinerama flag (in your .xsession) when starting i3, like so:
exec i3 --force-xinerama -V >>~/.i3/i3log 2>&1
…or use force_xinerama yes in your configuration file.
2. The explanation
Starting with version 3.ε, i3 uses the RandR (Rotate and Resize) API instead of Xinerama. The reason for this, is that RandR provides more information about your outputs and connected screens than Xinerama does. To be specific, the code which handled on-the-fly screen reconfiguration (meaning without restarting the X server) was a very messy heuristic and most of the time did not work correctly — that is just not possible with the little information Xinerama offers (just a list of screen resolutions, no identifiers for the screens or any additional information). Xinerama simply was not designed for dynamic configuration.
So RandR came along, as a more powerful alternative (RandR 1.2 to be specific). It offers all of Xinerama’s possibilities and lots more. Using the RandR API made our code much more robust and clean. Also, you can now reliably assign workspaces to output names instead of some rather unreliable screen identifier (position inside the list of screens, which could change, and so on…).
As RandR has been around for about three years as of this writing, it seemed like a very good idea to us, and it still is a very good one. What we did not expect, however, was the nVidia binary driver. It still does not support RandR (as of March 2010), even though nVidia has announced that it will support RandR eventually. What does this mean for you, if you are stuck with the binary driver for some reason (say the free drivers don’t work with your card)? First of all, you are stuck with TwinView and cannot use xrandr. While this ruins the user experience, the more grave problem is that the nVidia driver not only does not support dynamic configuration using RandR, it also does not expose correct multi-monitor information via the RandR API. So, in some setups, i3 will not find any screens; in others, it will find one large screen which actually contains both of your physical screens (but it will not know that these are two screens).
For this very reason, we decided to implement the following workaround: As long as the nVidia driver does not support RandR, an option called --force-xinerama is available in i3 (alternatively, you can use the force_xinerama configuration file directive). This option gets the list of screens once when starting, and never updates it. As the nVidia driver cannot do dynamic configuration anyways, this is not a big deal.
Also note that your output names are not descriptive (like HDMI1) when using Xinerama, instead they are counted up, starting at 0: xinerama-0, xinerama-1, …
3. See also
For more information on how to use multi-monitor setups, see the i3 User’s Guide.