1st – Are they going to be the same domain? Share files? Replicate databases? Share backups?
If you answered ‘yes’ more than once, then it is very straight forward. Make both Domain Controllers, let the AD replicate, and publish gloabal catalogue on both.
Probably would make most sense to have the respective users use the ‘local’ server for their network share/ user directory.
If you use ‘Roaming Profiles’ the users get their standard login even when logging in t the other office.