Our environment has 2 Windows domain controllers successfully replicating on W2k12. In that domain, like everyone else, we have tons of accounts, some of whose account attributes are required to be unique. For business reasons not worth visiting here, we need to sometimes swap attributes between accounts. That is, user1 may have (forced unique) mail of foo@domain.com, but we many need to swap that to bar@domain.com in order to give user2 mail attribute foo@domain.com.
If I execute an LDAP update to set user1 mail to bar@domain.com, get a successful result, and then assign foo@domain.com to user2, I am often (but not always) confronted with a uniqueness constraint violation. Even if I close sessions between operations; pause up to 2 seconds; etc. If I give it 5-10 seconds, it seems to go through OK, but I have an HTTP response hinging on this, so I'd really like to have bounced back with a result before then.
That leaves me two questions:
In retrospect, it's obvious that in order to check for uniqueness the AD would query the Global Catalog, requiring you to wait for replication with whichever Global Catalog server your DC is checking against. As you say, the solution is to perform the operation on a DC that itself hosts a copy of the Global Catalog.
In your particular scenario, with only two DCs, I would suggest that you make both of them Global Catalog servers. Quite apart from the renaming issue, you don't currently have any redundancy for operations that require a GC.