grammatical number - Using "do" or "does" for two things connected by "and"?

1. 
   
   • Does your mother and father know about this?
   
   
   • Do your mother and father know about this?
   
   


2. 
   
   • What does "this" and "that" mean?
   
   
   • What do "this" and "that" mean?
   
   
   

Which option is correct? I thought the second one of each example is correct because we are talking about plural things.

Answer

In both cases, the plural form of the verb is correct.

Do your mother and father know about this?
What do this and that mean?

This is because as you stated, the sentence is speaking of two things, so the plural is appropriate. The second sentence sounds a bit awkward because "those" would be more natural, but it is still grammatically correct.