can i work on the same iWeb on different Macs?

I used to do something like this with iTunes until they implemented alternative library selection. I'm 99% sure the same concept will apply. (Backup the files I'm about to mention before you try this). You'll have to have an external drive (flash drive is fine unless the site is huge) to store the 'Domain' file from iWeb. This is where the site content and layout is stored.

Take a look at this page:
http://macamour.com/blog/2009/01/15/transfer-an-iweb-website-from-a-mac-to-another/

I think that you can skip the copying step and just double click on the domain file from the flash drive in order to keep the drive's version the current one. Be sure to make a backup of the original before you try this