Although I don’t 100% understand what you are looking for, my first suggestion is:
Try checking “Ignore empty constraints”.
When you are building your search for your RG, build it so search on BOTH of the criteria you’re talking about. But also check the “Ignore empty constraints” checkbox.
This way, if they enter in a Customer value to search on AND a product, it will search for both. But if they enter in a Customer value and no product, it will search for the customer value.
Hopefully this helps.