Hello 

your problem based on _login_redirect( function you must override it. 

but as you said you need simple solution you can add this JS code to redirect and one register to other url 

odoo.define('website.user_custom_code', function (require) {
'use strict';

    $(".oe_signup_form button").click(function() {
     localStorage.setItem("from", "signup");
    });
    if(localStorage.getItem("from") == "signup" && window.location.href == "{YOUR ODOO URL }")

{
 
        $(location).prop('href', 'http://stackoverflow.com') // add your url here 
        localStorage.setItem("from", "");
    }
});