I actually have a similar problem. I’m just setting up WordPress after trying some solutions that were really more complex then I needed, so I’m still pretty new to the software.
I have three domains: a professional site, a political ranting site, and a personal site. I would like the first two to echo automaticaly to the third–posts at least, and if comments would copy as well I’d be overjoyed–and be able to post to the third site as well. I’m planning on doing a seperate install of wordpress for each, but haven’t decided yet if it should be one database or three. I suspect I can do this with triggers if the WordPress code wouldn’t be hopelessly confused, but I’m not sure if the trigger functionality in MySQL is advanced enough yet. Anyone have any ideas?