What is the best way of combining two companies separate Win2k domains (and hence separate forests)? Company X, which has a flat xxx.com domain, has purchased Company Y, which also has a flat yyy.com domain in a separate forest. All Company Y objects should be migrated to the xxx.com domain. Is creating a child domain such as yyy.xxx.com the best way forward?
You could create a yyy.xxx.com and then migrate the objects from the yyy.com to the yyy.xxx.com using ADMT. This would potentially produce the least amount of disruption. You would then also be able to fall back on the yyy.com domain structure if something went wrong with the yyy.xxx.com domain while you were performing the migration.
Of course, you should evaluate the need for the yyy.com (or resulting yyy.xxx.com at all). Is there a possibility...
that you could just move the objects into an OU instead? A lot of the times this is a possibility. Using OUs reduces the need for additional domain controllers and eases management of the entire structure.