I copied your spring security configuration and was able to reproduce the error.
Following line in the stacktrace gives the root cause of the exception:
at org.springframework.security.provisioning.InMemoryUserDetailsManager.createUser(InMemoryUserDetailsManager.java:59)
In InMemoryUserDetailsManager class, createUser() method, Spring checks if the user exists before creating an user. In your case you are adding two user with same username, hence there is an IllegalArgumentException.
I added two users with different usernames and it started working fine.