First off; do you need 2 domains? AD will cope happily with 1 domain spread over 2 sites, and it may reduce the complications further down the line.
You should set up your sites in AD first, specifying the IP address ranges in both. Then you can install the second DC and get them talking to each other. Once you give the 2nd DC its IP address within the correct range for its site, it will reside in that site.