Hi, Sophie,
depends how the questions are displayed.
I assume two single questions of type "numerical".
Well, enter in "Advanced settings" / "Question validation equation "
farm.NAOK>birth.NAOK with farm as code of question 2, and birth as code of quetion 1.
You also can enter an appropriate message in "Question validation tip"
See screenshots and don't mind the German texts.
Best regards
Joffm
Volunteers are not paid.
Not because they are worthless, but because they are priceless