You can start by taking a look at some of my examples for dynamically setting fields using jQuery and AJAX here https://github.com/Hube2/contact-form-7-dynamic-select-extension. I don’t have an example that does specifically what you’re looking to do, unless you want to use something like select fields for both fields. The basics will be the same, the differences are all due to different things being needed for each type of field.
