Where there's a will there's a way.
Really it depends on what you want to accomplish. It sounds like you have two SBS servers that are running seperate name spaces? You have a forest, the landlord has an SBS server with a forest of it's own. The domain becomes the security boundary between the two systems. Each user logs onto the domain of thier choice.
Subnetting... Subnetting can be done at the router level so that each domain is segmented. Access to resources on the other subnet would be rules created in the router...
If the two systems require the same subnet and mask, build the DNS and DHCP as a system seperate from the domains. Windows can run DNS and DHCP from non windows systems. This information needs to be programed into the DNS and DHCP systems.
If the systems are small enough, use host files on each workstation and on the server that describe the IP location of each system. Do not include the systems you don't want seen by a particuler server or workstation in the host file.
Those are a couple ways to do it that I can think of without more information on what you are actually trying to do.
Thank you for the assistance. Although the landlord decided he didn’t want the tenant my customer, to add a sbs to his network. ???? So he’s getting his own ISP. Once he does I’ll complete the job. I want to experiment more with this for future knowledge. I will take both answers into consideration while doing it and respond with what was actually done to make it work. Thanks!!