How do I offer two versions of a quiz in Storyline?

There's not only one way so it kind of depends on your specific goals, like if there are any overlapping questions. If you're essentially making two branches of questions, each one would get its own results slide.

Then, while you've left-clicked a Results slide, select LMS/LRS tracking from the Result Tools:

That allows you to choose one or more quizzes to report to the LMS. In the example above, 1.3 could be the results slide for the branch for someone taking the assessment for the first time, and 1.5 could be the results slide for the branch of repeat test-takes.

How users get the correct quiz will similarly depend on your implementation and limitations. Can there not be two buttons on the main screen and the user clicks one? If they can't be trusted with that, you need to identify some sort of criteria to separate them from each other. If the retake branch is shorter than the first-time branch, perhaps the retake button pops up a layer asking for a password before allowing someone to take the retake test. I would think standard practice is to just have two separate courses and assign the appropriate one to the learner.

KatHarper​: The 2-Results-slides method described by AndrewBlemings-​ won't work in this situation. 

As the Tracking Option window indicates, Storyline will report on whichever option is completed first. 

That's fine when the user is steered to completely separate quizzes. But if the user takes the 2nd quiz as a re-do because they failed the 1st one, Storyline will still report the results of the 1st quiz.

However, with a minor tweak, using two quizzes with separate Results slides will work. 

Use the dropdown in the RESULT TOOLS/DESIGN tab to indicate that the 1st quiz is a Pre-Check. 

Keep the 2nd quiz as a Final Assessment. 

Control navigation so that if the user fails the 1st quiz (aka, the Pre-Check), they can't access it again. Instead, steer them to the 2nd quiz. 

For publishing, include both the Pre-Check and the Final Assessment as tracking options.

Here's how that set-up works.

• If a user passes the Pre-Check, Storyline submits that score and marks the course as complete.
• If the user fails the Pre-Check, Storyline won't mark the course as complete until the Final Assessment.

For more details: PRIMER: Pretest in a Storyline Course | E-Learning Heroes 

I think the more common option is to have one quiz that consists of one or more Quiz Banks with randomized questions. However, there's no way to ensure that the questions drawn the first time won't be drawn the second time. But that, plus shuffling answers, is usually considered good enough.

For future reference, this Exchange Best Practices section is best for general questions about eLearning. You’re asking how to do something in the software. That type of question is a better fit in the Discuss Articulate Products section of the website. That's where those of us who regularly answer questions about how to use the software spend more of our time. And that's where more folks go to search for answers specifically related to Storyline, Rise, and other parts of Articulate 360.

Thank you both! This information is gold.