Allowing learners to choose the better of two results

I'm familiar with how to build a course that allows learners to retry a quiz, but what I need help with is how to allow the learner to choose the better of those two scores.

Here's my scenario:

• A learner takes a quiz with a set of questions pulled from a question bank.
  • If they achieve a passing score, they have the option to retry the quiz in an effort to get a better score.
  • If they do not achieve a passing score, they must retry the quiz.
• When retrying the quiz, the learner sees another set of questions pulled from the same question bank (some of which they have already seen in the first attempt, some of which will be new)

Since there is the possibility that the score on the second attempt is less than the first (e.g. a passing score on the first attempt, followed by a non-passing score on the second attempt), how can I configure Storyline to allow the learner to choose the better of the two scores, then, more importantly, pass that score to the LMS.  While I can write a trigger to adjust a results variable, results variables can't be manipulated in that way, and the LMS can only receive the results from one results slide, defined in the publish settings.

I'd appreciate you sharing any ideas you have on how to solve this.