I encountered the question, "Which trait does a savings account and a checking account have in common?" I suggested that it should read, "Which trait do a savings account and checking account have in common?" For which I was rebuffed. Shouldn't the helping verb do agree with the compound subject, the two accounts?
1 Answer
Your understanding is correct. Consider the parallel construction
Which date do Mike and Sally plan to choose for their wedding?
No one in their right mind would insist it should be "Which date does Mike and Sally plan to choose for their wedding?"
Rearrange the words of your sentence and it becomes clearer:
A savings account and [a] checking account do have which trait in common?
It's never going to be:
A savings account and [a] checking account does have which trait in common?
