Yes, there is a way to do it using a custom answer checker; however, this generally goes against the philosophy of most people who use webwork. (For instance, I wrote some custom answer checkers to do this and was told by many people that I should remove this "feature", so I did.)
Instead, consider using an all-or-nothing answer grader and withholding feedback so students won't know if their multiple choice answer is correct until both parts of the problem are answered correct:
$showPartialCorrectAnswers = 0;
If you really want to check whether your students know the answers to multiple choice questions, code them up separately and put them in a homework set that allows only one attempt per problem (as set in the Homework Sets Editor).
Another possibility would be to follow the multiple choice question by an essay question that gets graded manually, so that you're really probing them for something more than just surface-level knowledge.