One additional point might be relevant here:
If the couple own their properties as 'tenants in common' then each of them owns a distinct share in each property and is free to leave it to whomever they like.
However if they own their properties as 'joint tenants' then neither of them owns a distinct share in of any property. It's their 'partnership' which owns the whole of each property. When one of those partners dies, the whole of each property will
automatically belong to the other partner. Anything in a will which sought to achieve any other outcome would be invalid (because nobody can bequeath something which they don't actually own).
https://www.gov.uk/joint-property-ownership/overview