If you can do moderate PHP coding, there is a way, but it includes changing a file of the plugin, so if you ever update the plugin, you have to include this change again.
1. Set the "send to" of your form to b@mysite.com (default: not logged in)
2. Make a backup copy of plugin file contact-form-7/includes/classes.php in case something goes wrong.
3. In plugin file contact-form-7/includes/classes.php at around line 500 locate this:
$recipient = $this->replace_mail_tags( $mail_template['recipient'] );
4. Add a new line of code so it looks like this:
$recipient = $this->replace_mail_tags( $mail_template['recipient'] );
$recipient = apply_filters( 'ov3rfly_wpcf7_change_recipient', $recipient );
5. Then in your themes function.php add this:
function my_change_recipient( $recipient ) {
if ( is_user_logged_in() ) {
$recipient = 'a@mysite.com';
}
return $recipient;
}
add_filter( 'ov3rfly_wpcf7_change_recipient', 'my_change_recipient' );