Can't create user in SQL Server
It is normal you cannot associate two different user to a single login for a database. You must understand the difference between a login and a user.

A login authenticate you on the server. Once you are authenticated, the login will impersonate a user to access the specified database of the connection string.

So a login can have only one user per database.

Sample : login [myDomain\myUser] has access on SqlServer. This login can have a user per database : userA for databaseA, userB for databaseB.

If you want to add an other user to the login for the same database, you have to drop the first user. Instead, you have to create a different login for the second user.

Hope this help you.