I think the best way to handle this would be to include both the member number and URL field in the form. Then using client side validations you would apply either a required or regular expression validation to the select list, then using this validation as the trigger for two show regions around the text inputs. For one of them you would choose for 'if trigger is valid' to show and for the other you would choose to hide. The help documentation for Validation Toolkit describes more about the response validations and using them with the client validations to achieve these type of results. Please post back with any questions that you have about any of these.
