Has anyone discovered ...

That could be done by having the user select custom buttons for their responses. You could use a number variable to track whether at least 2 correct answers were selected. You'd also need to track whether the user selected a wrong answer. Then a trigger could select the "correct" or "incorrect" button for a hidden multiple-choice question in the background. That's what would be used for scoring.

These posts might help clarify what I mean by a hidden question slide:

• Give partial credit on a multiple-response question
•  TIP: Create a "Pick One" Question with Multiple Correct Answers