This means that you have two forests. You will have to create one new domain
in one forest and move objects from the domain you want to join to the
forest using ADMT tool
(http://www.microsoft.com/downloads/details.aspx?FamilyID=788975b1-5849-4707
-9817-8c9773c25c6c&DisplayLang=en)
You can't just merge two domains.
--
Regards
Matjaz Ladava, MCSE, MCSA, MVP
Microsoft MVP - Active Directory
(e-mail address removed), (e-mail address removed) http://ladava.com