Creating one domain with two sites with AD replication in Win2K3 server

I have two LAN’s each with their own domain and ip address range that are linked via a T1 WAN interface. My task is to collapse one domain and join the server from the collapsed domain to the domain I am keeping to create one domain with two sites.

What is the best approach to accomplish this task?

Eg. abcd.xxx.com xyz.abcd.xxx.com
xxx.xx.200.1-250 xxx.xx.202.1-250
DNS, DHCP, Act. Dir. DNS, Active Directory

Collapse domain xyz.abcd.xxx.com and join server from xyz to abcd.xxx.com.