For those who didn't catch it first time 'round...
Two Sovereign States ("A" & "B") enter into a treaty of union to create a single Sovereign State. ("C").
"C" enters into a treaty of union with a grouping of sovereign states ("Z").
Whilst a member of "Z", "C" repeals its founding treaty of union and reverts to "A" and "B".
Under such circumstances, any and all treaties undertaken by "C" in respect of "Z" apply equally to "A" and "B".
If a member of "Z" desires to exclude "A" from "Z", then the basis for exclusion applies equally to "B".
As "successor states", as opposed to "new member states", "A" & "B" would only require a majority of the 27(8) to approve continued EU membership in their revised constitutional form. No single member, e.g. Spain, would have a veto.
So don't go rushing down to the immobiliers just yet expecting to find a bargain or three.
Scots resident in the EU will lose the right of abode in the EU if their native Country secedes from the Union
The "Union" as per the above scenario would be of the European, rather than British, variety.