Would multisite and multisite privacy be a good way to create a password protected area?

Hi

I am not at all familiar with multisite but, having done some research, I think it might be the solution to a problem I raised in this topic:

https://premium.wpmudev.org/forums/topic/members-area-with-one-loginpassword-for-all-members

I need to password-protect a whole section of a website with a single password that will change annually (so mustn't be too time-consuming to change). It is a Members Area but my client does not want to go the obvious WordPress membership route of individual login details for each member.

The private area includes pages, downloads, custom posts and a forum (currently using bbpress).

Do you think I could move all these elements to a separate site on a multisite network (the other site on the network being the 'public' bits)? My understanding is the two sites would use the same theme and so would look the same, but because they are really different sites I could easily password protect the whole private site but not the public one.

I would be very interested to know if I've understood the whole multisite thing right!

many thanks
Mariette