Active Directory/Exchange Migration

I’m going to be starting a contract for a client that’s helping two credit unions merge. They both have separate AD domains and Exchange organizations. It’s 2003 across the board. They don’t want to merge one into the other so a third “fresh” forest/domain was created and they want both domains/org migrated into the fresh forest/domain. The client mentioned that they can’t use the AD migration tool in their particular situation but I’m not sure if they are correct about that.

Any ideas/tips and on how to pull this off? Thanks!