"Where do your grandfather and grandmother live?"
Translation:A nagypapád és a nagymamád hol lakik?
I can understand that, it is true (a little bit) in English as well. But in English, the rule is very simple: compound subjects are singular if they refer to one person or thing. For example, Green eggs and ham is my favorite lunch!
So, at least in English, grandfather and grandmother is plural and never singular. But in Hungarian, clearly the rule is very different, and seems to be more like you describe.
It's possible I missed it, but I don't remember the course ever telling us this fact.