Part two of my June work for the Uni/CSIRO was to modify compiz to support multiple input devices.
Get it now from git://people.freedesktop.org/~whot/compiz.git, checkout the mpx branch.
The current state: compiz core supports XI2, and the move, resize and annotate plugin support multiple devices. So you can wobble two windows at once if you feel like it - just for extra bling. Likewise, you can resize a window into 4 directions at once (I think so, only tested it with two).
The approach I took was fairly simple: in core, replace some functions (e.g. pushScreenGrab) with a device-specific one (pushDeviceGrab). This was done throughout the whole of compiz, and instead of core events, compiz now listens for XI events. In the plugins, replace the single variables with arrays/lists large enough for each device.
For the move/resize plugins I had to use an additional hack - the window decorator tells compiz over ICCCM that a resize/move operation has started. The ICCCM message doesn't include device ids and doesn't have any free bytes to stuff them in. The solution I found was to simply query all devices and find the one closest to the reported position - with the button 1 down.
In theory, the mpx-aware compiz should also work on a non-XI2 server but I haven't tested it.
The work is not finished with many details missing, but the core is there. I won't be able to continue on it as it was a project written under quite some time pressure and that time is up now.
I encourage you to take it and finish it off, it would be good to see a real window manager supporting XI2. If you have any questions about how to continue, pop me an email.