I have a Customer that has two locations. They have an SBS 2008 server running in each location as a DC for diff domains (ie. Bobs1.local and Bobs2.local). They now want to connect the two locations via VPN and use just the Bobs1.local domain. They want to remove active directory from the SBS 2008 server in the second location, join it to the Bobs1.local domain and then DCPromo it the make it a Domain Controller. The Bobs2.local domain is small enough that it is doable and it would make management easier and add redundancy. My question is can I have two SBS 2008 servers both running as Domain Controllers for the same Bobs1.local domain or are there some limitations in SBS 2008 that would prevent this?
SnapOverflow
You cannot have multiple Windows Small Business Server 2008 computers joined to the same Active Directory domain. This is a violation of the license agreement and will cause one (or both-- I haven't tried it to see) of the servers to shutdown regularly with license violation warnings.
If I were in your position I'd contact Microsoft re: transitioning the SBS licenses you have to full version licenses. In Windows SBS 2003 the offering was called a "Transition Pack". The Microsoft web site makes some references to such an offering for Windows SBS 2008 but I'm not finding good details about it... (a twisty maze of passages)
re: the "Premium Edition" entitlement
Windows Small Business Server 2008 Premium Edition comes with an entitlement to install a second Windows Server 2008 machine (which can be a domain controller in the SBS domain). That doesn't entitle you, however, to install Exchange or any of the other SBS Server components on that server computer (see http://technet.microsoft.com/en-us/library/dd469604(WS.10).aspx for details on the specific entitlement).
It looks like there's a specific procedure to follow when doing this with SBS, through the SBS-wizard interface. Hopefully this helps.
Also, if the second site is close enough to run the server over to the bobs1 DC, it's sometimes worth it to do the dcpromo that way, just to save time, depending on your setup.
Functions were added in SBS 2008 Premium to allow a multiple SBS servers in the domain. Prior versions of SBS do not allow a second SBS server in the domain, although additional member servers and (I believe) second DCs are allowable.
The Microsoft document linked in Kara's answer is part of the discussion on that feature. I don't think Exchange can be loaded on both servers. This document discusses in more detail what can and can't be done with SBS 2008 on multiple servers.
IMHO, since you have fewer than 75 users, having them split among two mail servers is overkill anyway. I suggest migrating the users and shared files from one domain/server to the other, removing the second one, installing the second SBS server from scratch at the second location as a DC and file/print server only, and moving the necessary shared files back.