Formal adoption did not exist before 1927, so this would have been a private arrangement.

After 1927, it would have been unlikely that a formally adopted child would have given their birth surname as an alternative one at marriage as the whole point of the process was to expunge the old identity and assign a new one. A more likely reason for the alternate name would be if a child was known by his stepfather's name but no adoption had taken place so his "real" name (I.e. the one on his birth certificate) was different