what would you use the 2nd server for? AFAIK SBS2003 doesn't work with the old "BDC" concept. SBS2003 will only work as a sole domain controller.
a subnet does not equal a domain. a domain is a security boundary. a subnet is a series of ip addresses. you communicate cross domain if you log into that resource (each resource has a list of users it will accept).
so, for eg.
i can have a windows nt 4.0 server at ip address 192.168.1.41 and a SBS2003 server at ip address 192.168.1.100
if you set up a share on the sbs2003 server in domain "sbs2003" then you go to the windows 4.0 server, go to network neighborhood and search entire network, then go to workgroup "sbs2003" then click on the share you just created. you will then see a dialogue box that will allow you to log in to the shared resource.