Two Active Directory Accounts

Hi I have a user that has two active directory accounts, in different OU's (they have different user logon names)The problem is that now he wants a mac, so i setup the ad binding in 10.4.11 at the login prompt i type hes logon user name and password. Then im asked to choose which one of these two accounts , so i select the one i want him to login as. and it fails. i have reset the password on both accounts in AD. the other account i can login fine but not the account i want to, this works fine on the pc. any ideas thanks mark