|I had to chime in here. |
Be aware that there can only be one SBS server on a domain. If you add another SBS server to an existing SBS domain, they will begin shutting down every hour after 7 days. This is to allow for Migration scenarios.
You can, however, add multiple domains to Exchange 2007/SBS2008, but only one can be the primary and this is what all users will use as their default "send as" domain. So while you can have both upstairs.com and downstairs.com on the same Exchange/SBS server, and receive mail properly for both (assuming you have the proper MX records configured) your users will still end up as sending mail from either firstname.lastname@example.org or email@example.com depending on the global primary setting.
Another problem in this scenario is that even though they are two separate companies, they need access to the same data. This is the hard part as SBS does not support either implicit or explicit trusts with other domains or forests. Does either company have to actively modify data from the other, or is downstairs.com simply going to be using the data from upstairs.com as a historical reference.
If both need to modify data, you may have to look at some sort of cloud solution for data storage that can be accessed by both companies regardless of their domain membership. If it is only for archival purposes, then you can have 2 separate companies, each with their own server, network, and simply sharing an internet connection.
Let me know your thoughts or if you need more clarification.
IT Desktop & Network Consultant - MOS Master Certified, MCP, MCSA, MCITP - Windows 7, CCNA Certificate Pending, A+, Network +