Attestation

To not allow completion unless both boxes are checked, you will need to add another slide beyond that page. Use triggers so that the next button is not available until both boxes are checked (I would actually just do this with checkbox input interactions, not a survey question - you can control with triggers more easily this way). Then, use getting to the very last slide as the completion trigger. That way they can't mark as complete until having checked both boxes. As you said, that last slide can just have basic "thank you, you may now exit" text on it.