$itemOptions = $item->getProduct()->getTypeInstance(true)->getOrderOptions($item->getProduct());
$addProductSku = 'Silky-#1-000';
                        $options = array(
                                'product' => Mage::getModel('catalog/product')->getIdBySku($addProductSku),
                                'super_attribute' => $itemOptions['info_buyRequest']['super_attribute'],
                                'qty' => $item->getQty(),
                            );
                        $product = Mage::getModel('catalog/product')->loadByAttribute('sku',$addProductSku);
                        if ($product -> isSaleable()) {
                            $cart -> addProduct($product, $options);
                            $cart->save();
                            $session->setCartWasUpdated(true); // Not really necessary - Vicary

                        }

Invalid method Varien_Object::checkQtyIncrements(Array
(
    [0] => 1
)