You can have both iMovie 6 and iMovie 9 (11) installed.
Happily this is easily achieved, but it has to be done backwards.
Delete iMovie 11 (just drag it to the trash). Now install iMovie 6 from the iLife 6 install disks.
Now re-install iMovie 11. This automatically moves iMovie 6 into a folder it creates in your Applications folder called iMovie Previous Version.
Then reinstall iMovie 11.
Now you have them both, and you can have both in the Dock as well.
Alternatively you could try this method, suggested by poster Uitech:
Open a terminal and type "touch /Library/Preferences/com.apple.iLife011.plist"
(No quotes)
Install iMovieHD6 from the iLife 6 install disk.
This works with both Leopard and Snow Leopard Macs.