…it was only common courtesy to keep his knowledge to himself… he added that there were two entries to the house…
as he spoke to Winston at the beginning of chapter 4?
Mr. Charrington was assuring Winston that there was no reason for he and Julia to worry about the front door not working since there were two doors to use for entrance to the house.
Mr. Charrington was assuring Winston (in a round about matter) that there was no need for him to worry about being turned over to the Party because he felt like everyone should have their privacy to do what they please in their lives.
Mr. Charrington wanted to make Winston aware that there was an alternate route for escape should the Party invade the privacy.