We had the same problem! You can use the multi step form with WP SMTP Plugin but you’ve to delete or uncomment the following lines:
{Pluginfolder}/includes/lib/msf-shortcode.class.php Lines 333-336
if ( count( $email ) == 1 && $cc === 'on' ) {
// TODO: Is this really right?
$copy = wp_mail( $email, 'CC: ' . $settings['subject'], $content, $headers );
}