This is now not necessary as Ubuntu 10.04 has dual pane built-in, just hit F3
Found this on OMGBuntu and have been using it, its pretty useful so far and has been stable.
sudo add-apt-repository ppa:berndth/ppaThen I opened up Synaptic and clicked "Reload" to reload all repository data, and "Mark all Upgrades" and apply it. Alternatively you can do:
sudo apt-get update &&;sudo apt-get dist-upgrade
You will need to either logout and back in, or hit ALT+F2 and "killall nautilus" and then ALT+F2 again and "nautilus" to restart it with the new changes. Note that if you have added extra buttons to the Nautilus toolbar you will have to readd those as the update will have over written them. Add the changes to the NEW file, don't just copy over the old if you had a backup. The new file contains menu items for the split pane which you won't want to lose.
Now you can use F3 to pop open or close the extra pane.