Comment 17 for bug 430919

To the extent of my knowledge, none of the open source video drivers support multi-card. See bug #316514 which is the primary master bug for this issue. It sounds like this bug report is essentially a dupe of that one.

The closed source binary -nvidia driver supports this configuration via Xinerama. Again, look to the support fora for assistance in making that work. Regarding the -nvidia driver stability, I acknowledge that it can be hit or miss; we conducted a concerted QA effort around it in Lucid and found it was generally solid for many users. Ultimately though, even if we find bugs in the closed source drivers pre-release there's often not much we can do about that at the distro level. Perhaps the newer -nvidia in maverick will give you better luck, or you could use one of the (unsupported) versions from nvidia's website.

Again though, this is treading into the realm of technical support so I'd encourage you to seek help in the nvidia user's forums or askubuntu.com or similar.