during the 18th century to address each other when married by this formal manner.

In JA's books it is usually a habit adopted by people of an older gernation, or it is used in her earlier works.

For example, in P+P we have Mr and Mrs Bennet who always address each other formally,but in Emma, written much later, Mr Knightley asks Emma if she will address him as "George",which seems to suggest that by that time it was perfectly acceptable for married coples to address each other so( the beautifully correct Emma, true daughter of the attentively formal Mr Woodhouse,refuses so to do)Certainly John Knightley addresses his wife by her first name.;-)