First the unfortunate news. Exchange2000 Organizations are bound by an AD forest. So since your two AD domains are seperate, even though the Organization names are the same, they will never be in the same organization (because your AD forest, is the organization boundry).
Since Exchange2000 uses AD for its directory service, AD is the replication piece, not Exchange (like in 5.5).
Good News- InterOrg/InterForest (same terms when speaking AD/Ex2K) can be done with the Microsoft Metadirectory Service (MMS).
An Interforest Solution
None of the current Microsoft interorg solutions for synchronizing mailbox information between Exchange Server 5.5 organizations work with Exchange 2000 because Exchange 2000 uses AD. The interorg tools that work with Exchange Server 5.5 depend on a Directory API (DAPI) interface to the directory or work only with the Exchange Server 5.5 DS. Win2K and Exchange 2000 include the Active Directory Connector (ADC), which uses LDAP to synchronize between Exchange Server 5.5 and AD, but the ADC currently doesn't support interforest synchronization.
Although not really a directory synchronization utility, MMS can combine the mailbox-enabled users and groups native to each Exchange 2000 organization into one directory (the MMS metaverse) while keeping track of which organization owns each directory entry. MMS can update any of the connected forests with entries external to the local Win2K forest namespace so that those users show up in the Exchange 2000 GAL. Through attribute flow, administrators have granular control over which attributes flow between the forests to support the business needs of each organization. In the near future, Microsoft will add an interforest synchronization wizard to help you configure MMS. http://www.microsoft.com/windows2000/guide/server/features/mms.asp
