If we need to add the related products for the single product we go for a Bundle products feature. The Bundle products have one parent product and related child products. Whenever we add a Parent product from the opportunity the related child products automatically added to Opportunity. Salesforce has tried this feature and let us know if you have any query.
1. Parent product,
2. Parent product having the child product
3. Go to the opportunity product and add the product
4. Choose the bundle product
Let us know if you have any queries!