Is it possible to support two different attributes "mail" and "uid" as user name based on the logic "userNameSameAsEmailFlag=N"?

Out of the box it is not possible to configured CT users to essentially have two user names. You could write an Authentication and Authorization hook to authenticate the user with an alternate logon type by making direct ldap calls within the hook to lookup the correct CT username based on the alternate name.

After further discussion the customer asked if he could have some users logon with one username format and others with another. I told him that this is possible, he would have to configure CT to use a "customUserName" ldap attribute where the users CT username would exist in one of the two formats.