Safari error in Intranet Http & Https
I'm not sure about what you are consider as multiple login. 

Anyway, the difference between the two protocol is that: HTTPS is the standard HTTP with additional S a secure layer. This additional layer is used to perform an encrypted connection and so the link is more secure. The main motivation for HTTPS is authentication of the visited website and protection of the privacy and integrity of the exchanged data.

The difference in your intranet company is that with HTTPS you don't need to verified the login multiple time because you are already secure (the server already know you), while from the HTTP you have no encryption and so every time you must autenticate yourself.