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?
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Your understanding is correct. Consider the parallel construction
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:
It's never going to be: