Looking for Recommendation - Two Logical Domains on One Network

I have a small network of 40 users running Windows 2003 Server and
Active Directory. They all run under the logical and Internet domain
of abc.org. Now I want to add another group to this same physical
network of about 20 users with the logical domain of xyz.org.

Thanks to what I have learned on this newsgroup, it seems I have two
ways to do this:
1) Promote a member server on my domain to a domain controller, add
xyz.org as a new tree or non-contiguous child domain.
2) Create a new OU for the xyz.org and give them a different UPN
suffix.

First question: is this correct and do I have any other options?

Second question: what are the ramifications for: DHCP, File sharing,
Exchange email, security? For example, it might be nice to give
abc.org users one DHCP block of addresses, and xyz.org a different
one. I would like abc.org users to see one network neighborhood and
xyz.org a different one, etc.

Thanks in advance for any responses.

 

Different subnet, no WINS, will ensure they can't browse.
The answer to your questions about the right way to do this is: it depends
what you want to achieve. How different are these two organisations?
Anthony