I replied to your question in the blog comments but I'll repeat it here so everyone can see it.

Do you mean two separate domains (http://www.mydomain.com and http://www.yourdomain.com) or do you mean two sub-domains (http://myspace.domain.com and http://yourspace.domain.com)? If they're sub domains then it should work just by pointing to the provider to domain.com though it's been reported that synchronization doesn't work properly in that case (it's on my list of things to do). If they're two separate domains.... well, I don't know if that can be done at all. If it can be it'll require some heavy duty changes to the code as the provider would have to check more than one Active Directory.

I guess what's really needed is a better description of what you're trying to accomplish (ie: the physical layout of the different domains, servers, etc.).