We are currently developing 3 sites (including Commerce) which all sit under the same company, but are independent sites. We have been looking in to cross site login to Edit mode, and were wondering if it was possible to have a single, shared database for the editors security, while keeping Customer logins within their own sites. If so, how do we go about doing it?
You could look into the mutliplexing provider that comes out the box with EPiServer. I never tried this specific config but you could mutliplex the individual site DB's with a shared ASP.net membership and role database for common access to edit mode.
If you are using newer versions of CMS/Commerce then you could also consider using federating security.