If you share it with only your friends, that does not mean "only with this friend, not the other friend". Both friends are "friends". So they both have access.
If they want to leave a personal comment on your profile, you and others know it's visible to other friends. The privacy is not there, it's there to the "friends" group. Not beyond.
If you want a personal comment between just you and another person, and optionally one or more exclusive friends, a personal conversation or conference is the feature to use.
A public profile is public, and not a personal conversation between friends in threaded profile mode.