Filter Dropdown with two objects from form?

I have a tax rate object that is linked to a province object and a year object, giving me the tax rates for the forms current province and current year. However in the form when I edit the input section for the field, I can only pick one of the filtering options, year or province.. How would I go about filtering on both, so that I have just the tax rates for that year and province?

Pretty difficult to know without seeing some of it. Can you share some screen shots?