Yes, you are missing something, but I would not call it obvious by any means. The data for your site(s) that iWeb uses for editing is stored in a file called Domain.sites (or Domain.sites2 in some cases).
That file by default is located by the following path on your hard drive:
Users/username/Library/Application Support/iWeb/Domain.sites
To edit the same site on a second Mac, the Domain.sites file needs to be copied to the second Mac, preferably to the same location for simplicity. Keep in mind that if you edit this file on say Mac A, you would need to copy the file to Mac B if you wanted to continue edits to it there, then back to Mac A if you wanted to further edit it there. For that reason, it is probably simplest to try to do all edits to a particular site on a single Mac, although....
....one suggestion that I know some people have used successfully with multiple Macs editing the same Domain.sites file when they have MobileMe is as follows.
• Initially copy the Domain.sites file to your MobileMe storage space.
• If you want to edit it from either Mac, download the file and put it in place on the Mac you wish to edit with
• Make edits, then immediately after quitting iWeb, copy the file back to your MobileMe space
• Repeat as desired
That will work ok since you will always keep the newest version of Domain.sites on your MobileMe space, but only if you understand that it can only wind up being edited on one Mac at a time successfully.