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 are then redirected to your products, and you will notice that the secondary product can no longer be published on your site; however, you can still edit it to modify it.