I have found a nice little piece if software that will allow you to manage Multiple iWeb sites via the ability to have more than one domain file. There have been a few different applications that have done this but they are overpriced in my option. The application I am presenting to you today is iWebSites. The best part is it is Donation ware. (Please if you like the software make a donation to the starving programmers who made this nice piece of software) The way it works is it allows you to open each site as a different .domain file which is how iWeb stores your sites.
Thanks to Cool OSX Apps for the link to the software
Give this a shot and let us know how it works for you.