Dynamic Drop Down allows users to select invalid combinations

Hello! I have a spreadsheet here which uses a dynamic drop down list. The user selects a customer in drop-down 1. Drop down 2 displays a list of products to choose from which are specific to the customer selected in the first. My issue is that if Customer 1 (for example) and one of their products is selected, the user is able to go back into the first dropdown and change it to Customer 2, and the value in the second drop down is still allowed to be a Customer 1 specific product. I think I could get around this with a macro, but I'm not amazing at them and I can't get it to work. Any ideas would be greatly appreciated Thanks in advance