Make user’s first and last name as user slug

To update existing users, try to make a script file and put that code on it (you have to require necessary file wp-load.php if you put that file in the root of your wp instance) or by listning on a hook like init hook :

$blogusers = get_users( 'role=subscriber' ); //get users by role
// Array of WP_User objects.
foreach ( $blogusers as $user ) { //loop throught users
    update_user_meta($user->ID, 'user_login', sanitize_title($user->first_name.' '.$user->last_name)); //update user login

    //if $user->first_name / $user->last_name didn't work, try to get the first and last names by using get_user_meta($user->ID, 'first_name', true)...
}

if you wanna use that behaviour just after user has registred, you have to use the user_register hook, and then put the same code above in your function…

PS : Code not tested.