code: public function setAlternativeAccount($iban1, $swift1 = null, $iban2 = null, $swift2 = null)
      {
          if ($swift1 !== null) {
              $iban1 .= '+' . $swift1;
          }
          if ($iban2 !== null) {
              if ($swift2 !== null) {
                  $iban2 .= '+' . $swift2;
              }
              $iban1 .= ',' . $iban2;
          }
  
          return $this->add('ALT-ACC', $iban1);
      }
docs: Set alternative accounts
  
  @param string $iban1 Account 1 - IBAN
  @param string|null $swift1 Account 1 - SWIFT
  @param string|null $iban2 Account 2 - IBAN
  @param string|null $swift2 Account 2 - SWIFT
  @return QRPayment
  @throws RuntimeException