Separate quiz review slides for correct/incorrect?

Yes, it is possible. And you don't need to use variables. 

When a question slide has a layer called "Review," the program will automatically show that layer during a review.

• Storyline automatically creates that layer if you enter text in the Post-quiz review field in Form View.

• Or you can add a layer yourself, and name it "Review"  (use that exact name).

On that layer, you'll need to provide both versions of the feedback. I think the easiest method is to use separate states (e.g., "correct" and "incorrect") of the same object. Here's how to show the appropriate message during review.

For a Multiple Choice question, add a trigger that changes the object to the "correct" state when the timeline of the layer starts with the condition that the correct response button = Selected. Add an Else statement that changes the object to the "incorrect" state. 

For a Multiple Response question, the trigger needs a condition for each response. Note: for the wrong answers, use the condition "≠ Selected." (All built-in states are built on Normal, so "=Normal" doesn't work well in these situations.)

This is demonstrated in the attached file.

FYI: It is worth the effort to learn about variables and trigger conditions, because they provide the real power in Storyline. Here’s more information:

• This post provides an overview of variables. It also links to numerous resources. The Value of Variables | Articulate - Community
• The User Guide section about triggers includes info about conditions.  Storyline 360: Working with Triggers | Articulate - Community