In the Flex theme, you select your default product page template within the Theme Editor (customize) under Theme Settings > Layout > Templates. Keep in mind, the content blocks configured in the theme editor, will show up for all products using that template.
In order to show unique content blocks for the same template, you would need to duplicate the template.
Current Theme Versions (Online Store 2.0)
Since all Flex's product page templates are converted to JSON for Online Store 2.0, this template can be easily duplicated through the theme editor.
In the theme editor (Customize), select Products then [+] Create template with the page selector.
Give the second template a Name, then change the Based on setting to scrolling.
Select Create template to complete the duplication.
Assign Templates to Batches of Products with Bulk Editing
Vintage Themes (Pre-Online Store 2.0)
Step 1: Head into Online Store > Themes > Actions > Edit Code.
Step 2: Under Templates click Add a new template.
Step 3: Make this new template of the 'product' variety, call it 'scrolling-2' and then click 'Create template'.
Important: You must start the name you enter here with the word "scrolling" in order for all features to work. For example, entering "scrolling-2" (which becomes "product.srolling-2.liquid") or "scrolling-new-product" (which becomes "product.scrolling-new-product.liquid") will work, but "new-product" (which becomes "product.new-product.liquid") will not.
Edit the template code
Step 4: Remove all the code that is in the file you just created and replace it with the code from the original product-scrolling.liquid template file.
Step 5: Near the bottom of the product.scrolling-2.liquid file, you'll see this code:
{% section 'product-image-scroll__main' %}
Step 6: You'll just need to update it to be:
{% section 'product-image-scroll__main-2' %}
Step 7: Save your template!
Edit the section code
Step 8: Under Section click Add a new section.
Step 9: Call this new section 'product-image-scroll__main-2' and click 'Create section'.
Step 10: Remove all the code that is in the file you just created and replace it with the code from the original product-image-scroll__main.liquid section file.
Step 11: Save your section.
Assigning the duplicate template to your product
Step 12: Once you've duplicated the files, you'll need to assign the new template to your product! In the Shopify Admin > Head into Products and locate your product. Under Theme Templates, select 'product.scrolling-2' and save.
This process can be repeated if you need by creating more templates and sections and following the naming conventions as the above instruction, such as:
Template: product.scrolling-3
Section: product-image-scroll__main-3
...and so on