Dual monitors (Twinview) works with Dapper and XGL for an Nvidia Card.
Instead of using Dapper's repository for compiz, I pointed to Quinn's repository. I mixed the instructions from http://www.ubuntuforums.org/showthread.php?t=131267&page=4 and https://wiki.ubuntu.com/CompositeManager/Xgl
You need the Universe and Multiverse repositories as well as either one of the following two non-Ubuntu repositories:deb http://www.beerorkid.com/compiz/ dapper main
deb http://xgl.compiz.info/ dapper main
If you need help with adding repositories, please see AddingRepositoriesHowto.
The packages in Quinn's compiz repositories can be gpg authenticated. If you want to be able to check the packages against QuinnStorm's gpg signature, import her key by typing the following command in a terminal:gpg --keyserver subkeys.pgp.net --recv-keys 0x31a5f97fed8a569e
gpg --export --armor 0x31a5f97fed8a569e | sudo apt-key add -
Upgrade all the packages that Quinn's repository suggests and reboot. Currently I am using the forums suggested thefuture script, but I will be modifying and editing the ubuntu wiki page listed above once it all settles down.