Mary's marriage treaty in otl was quite clear - her issue would succeed in order - there was no suggestion of dividing the crown - so if she has a son he will be Dauphin of France and Duke of Rothesay and on the death of his parents King of France and Scotland. If she has a daughter that daughter will be eligible to succeed as Queen of Scots but not of France.
The secret clauses promised the Scots succession to the descendants of Henri II of France if she died without issue - though it is pretty likely it wouldn't have been accepted in Scotland.
The secret clauses promised the Scots succession to the descendants of Henri II of France if she died without issue - though it is pretty likely it wouldn't have been accepted in Scotland.
So if Mary of Scots is married to Francis or Charles and they have more than one son, would the younger one be designated for Scottish throne?