Two Login Forms for the Site in Drupal 7
If you are talking about a single account that everyone who uses the site will use, the security implications are horrible. In the context of a business site, what happens if an employee is terminated. Does everyone have to learn a new password. What about the case of one disgruntled user who does something malicious? Do you want to have to track responsibility by back-tracing an IP address? There are so many reasons not to implement this requirement.

Drupal looks up password by user (user name or unique email) and being able to attribute site changes to an individual user, and to control access by individual user is extremely important with most sites. To deviate from the tried and true method will require a custom module, and I seriously doubt it is a good idea simply to alter the user experience.