If you have DataAssist then there is an option to store the newly inserted ID as a session variable. If it matches the login session variable you are using, it is as simple as updating the session variable name in the insert to match the login.
If you are storing more than one session variable at login, and you own ecart or Cookies toolkit, then you will have a set session value server behavior and you can set the session variables that way.
You could probably remove the redirect page from your insert and apply the server behavior to login on the register page as well.
At the end of the day, Login really means setting session variables to match user information, usually at least the ID and sometimes more. Then rules are created to check those values and respond with conditional content or redirects.
At the end of the day all you need to do is set the same session variables that your login sets and they will be logged in. There are quite a few ways that could be done.
