ADMT works because when running it from the command line you use the RUNAS command to authenticate to the other domain. Similarly, you must syncronize passwords between the two domain administrator users.
But here is the real answer to your question. If you get what you want to work it is not legal according to Microsoft’s acceptible use policy. SBS must be the only DC and all products must be installed onto it. You can only add other separately licensed products to the domain.
A typical SBS network for a larger amount of users would look like this:
SBS 2003 server: AD, Exchange, ISA, SQL
Windows 2003 Storage Server: File and Print
Windows 2003 Terminal Server: Remote users and single application publishing
This can be a great setup, however, keep your SBS server healthy because rebuilding it can be a difficult. I never recommend installing any third party applications onto the SBS server to clients. Why? Because Microsoft’s SBS service packs often contain more than fixes and this can break 3rd party software. It is best to move them to small individual servers or virtual machines.