Instructions say to not assume a surname just because you see the surname of a spouse. What about a record tax record that shows the full name of one spouse, but also shows "ux" or "wife" and then lists the first name of the other spouse? Do we do control blank for the surname of the added spouse?
If it said John and Mary Smith, you would enter it for both. If it says John Smith and his wife Mary, only John would get the surname.0
If you are working on US, Massachusetts—Boston Tax Records, 1822–1918 then you should check example #3 in the project instructions. It will answer your question.
It shows that you do index the surname.
Hope this is what you were asking.