have you configured BOTH the ACS to back-end with BOTH the LDAP servers? if yes, then the LDAP server listed first is the one where you are getting autheticated succesfully.
The reason is because, when authentication request is sent to the first LDAP server (assuming the user is in the 2nd LDAP server), the 1st LDAP server replied with a FAIL message back to ACS, and the ACS fails this attempt. ACS will not send this request to the 2nd LDAP server, because the reply back from the 1st LDAP was FAIL and not an ERROR. The 2nd LDAP server will only be contacted if the 1st LDAP server is not responding.
HTH
R/Yusuf