We are in the planning process of a site with similar specs. We plan to use two siteaccesses, and one database. Both siteaccesses will use the same database in their settings files, but all other settings will be independent. Can anyone else confirm that this will work as intended?
The questions I have seen raised are in regards to search results, direct links into pages (templates not knowing which design to present with the content).
It should work without any major trouble as long as you don't expect to have true seperation with regards to templating priviledges (content administration isn't much to worry about).
As for chosing the right template, that's inherently done based on the siteaccess, so no sweat there at all. Depending on the size of your site performance could be better with seperate installations, obviously.