I would STOP trying to combine the two domains with the same name immediately!!!
So the duplicate domain names are a no-no and can not reside in the same forest I would use adduses.exe that can be found on a windows 2000 resource cd (or on the web) This will allow you to export your users to a delimited file and then use the same exe to import them to your AD forest of choice.
Passwords do not come over but using a program like l0phtcrack you can retrieve and import them as well. Or just require all users to have to change their password on the next login.
Microsoft first introduced this app in NT and you can read about it here.
Let me know more details if you want further help!