OK I've spent hours reading through the various shared database multisite options including TAC, multi-domain, OG, Domain-access and a number of others. Many of them seem to be very much concerned with access control for a community of logged-in users. One thing I'm not seeing clearly is the ability of a solution that allows a good old fashioned Web 1.0 webmaster to control exactly what content goes to which site.
By this I mean that for a casual visitors, google bots, etc
- Story A is on site1.com
- Story B is on site2.com
- Story C is on site1.com and site site2.com
Ideally this would be set by the 'webmaster' when they are entering the story. The important thing would be that Story B doesn't appear anywhere on site1.com, not on any list of stories or any menu on site1. Even if a you manually changed the URL to from site2.com to site1.com you still couldn't find story B on site1.com (hopefully you would get a custom 'Page Not Found' if you tried that).
I have the feeling that something like this may exist already but that it is hidden behind all the descripions of fine grained access control. Multi-Domain seems like it might be close to what I'm looking for but I'm a bit uncertain about the 'small patch to Drupal core',reliance on 'single signon' and the lack of activity.