If you want to merge two products into one (e.g., you want to customize a single product with two different visuals), it's very simple:
You must first create the two products by selecting for each the colors related to the visual (e.g., a white pattern for dark-colored t-shirts and a black pattern for light-colored t-shirts)
Then go to your list of created products
Choose the first product, click on "Options" then on "Merge."
Your second product will appear in the list of compatible products on the right side of the screen
Once selected, click on "Merge with secondary product"
You'll then be redirected to your products, and you'll see that the secondary product no longer appears, but is integrated into the first product.
Now, to check that the merge has worked, simply open the product and click on “View” (or “Preview” if it's not published) in the top right-hand corner. You'll be able to admire your new product, which reveals two superb visuals.