There is a bug in the current version of Inventory (1.1 release 2) which prevents the use of multiple local credentials with the same name. So, if you have two users called "Administrator" then only one of them will be usable. This will be fixed in a future update but to get around this you can use the ".domain" format for the user name. For example:
Each of the domains begins with a . so they are both treated as local users but they will be kept distinct within the application and not get mixed up.