I guess it depends on what you are trying to achieve in the test (basically what part of the system you want to exercise front end, back end, middle ware). I have run test using one log in and password for 500 VUs and had no problems.
I would say the simplest way to check for data contention is to have 2 VUs login using the same user id and password.
If your developer does not know if this is possible I would send up some red flags. In a production environment allowing the same user to login with the same user name and pass word would be a security violation for a lot of companies especially if they are dealing with HIPAA or other sensitive information.
But in the end doesn't it all come down to BEER? Beer is the ultimate answer to all questions in the universe so yes the answer to your question is BEER.