As I understand it, "true" mirror wills are now a rare thing because they do tie both parties into doing certain things and IIRC, one will can't be changed without changing the other. Identical wills (which is what my late DH and I had) are just wills which say the same thing, the only differences being the name in such clauses as "all to partner". Either will can be changed at any time and the survivor's will remains in force after the death of their partner.....but I am not an expert and you really need Barmaid.