I have two clients sharing one building, each with their own exchange 2010 server. Some employees cross over working for both companies.
The servers are linked in that you can view either exchange server from each others console.
For the cross over employees if they have an email address at both companies in the same outlook profile there are issues with public folder contacts. They can see the entire contact list but the contacts only show the persons name. All the details such as email address, phone, etc. won't show unless you have that email address on a separate profile.