Progressive disclosure login and security

More and more web sites are using the UX pattern "progressive disclosure" for login purposes. I understand that this is for better user experience and cleaner user interfaces.

Since I use my browser (Firefox, IE) to save passwords for many web-logins, I'd like to know if "progressive disclosure" improves or declines security of the login process.

• Are there two seperate entries stored in the browser (login name and password) or does the browser still understand that this is a single login with a login name and a password?
• Do browsers have special features to handle progressive disclosure during login?
• As a developer: Do I have to cheat the browser to tell this is one login form?