Button Stack

If the two calculation lessons are the same length, you could track completion by requiring the user to go through the % of the course that would include any shared lessons + 1 calculation method. 

Here's another option: At the end of each calculation lesson, include a button that jumps to one final "You're done!" lesson. In that lesson, insert a Storyline block with a completion trigger. Track completion of the Rise course with the Storyline block. Here's more info: Primer: Storyline-block completions in Rise | Articulate - Community 

By the way, to avoid confusion about what needs to be completed, consider hiding the navigation menu. Use button blocks to move through lessons instead.