For registration to be your homepage you need to create a menu item for it then set it as your homepage. However, users that are logged in viewing the page will see an error that they're already logged in. So you'll likely need to use Joomla Guest access usage so that homepage is only accessible to Guest users, but I'm unsure if Joomla will let you have more than 1 homepage in this case even when using the Guest access usage. Please see the below Joomla tutorial for hiding something from logged in users.
docs.joomla.org/How_do_you_hide_something_from_logged_in_users%3F