Magento multiple order on one checkout or order splitting(Vendor wise order splitting)

I had to split orders based on the vendor/seller selling that product.

Like, for example, if someone orders Product1, Product2 and Product3, where Product1 and Product2 are sold by Vendor1 and Product3 is sold by Vendor2.

Then, 2 orders with different Order Ids must be created where the Order1 will have details regarding the order of Product1 and Product2 and the Order2 will have the details regarding the order of Product3.

I changed the saveOrder() function. It now splits the order by fallowing ways.

1.Create config file or you can use existing module configfile.

<?xml version="1.0"?>
<config>
    <modules>
        <Pawan_Splitorder>
            <version>0.1.0</version>
        </Pawan_Splitorder>
    </modules>
    <global>
        <helpers>
            <splitorder>
                <class>Pawan_Splitorder_Helper</class>
            </splitorder>
        </helpers>
        <models>
            <splitorder>
                <class>Pawan_Splitorder_Model</class>
                <resourceModel>splitorder_mysql4</resourceModel>
            </splitorder>
            <checkout>
                <rewrite>
 <type_onepage>Pawan_Splitorder_Model_Checkout_Type_Onepage</type_onepage>
                </rewrite>
            </checkout>
        </models>
    </global>
</config> 

2.Next create rewrite model file and you can extend parent Mage_Checkout_Model_Type_Onepage  in this file saveOrder() function rewriting.

public function saveOrder()
    {
        $quote = $this->getQuote();
        //First build an array with the items split by category
        $sortedItems = array();
        foreach($quote->getAllItems() as $item)
        {
            $seller = Mage::getModel('catalog/product')->load($item->getProduct()->getId())->getFilternow();
            $product = Mage::getModel('catalog/product')->load($item->getProduct()->getId());
            $sortedItems[$product->getFilternow()][] = $item;//array('product_id'=>$item->getProduct()->getId(),'qty'=>$item->getQty());
        }
        $allOrderId='';
        foreach ($sortedItems as $seller => $items) {
            foreach ($quote->getAllItems() as $item) {
                $quote->getItemsCollection()->removeItemByKey($item->getId());
            }
            foreach ($items as $item) {
                $quote->getItemsCollection()->addItem($item);
            }
            // Update totals for seller
            $cart = Mage::getSingleton('checkout/session')->getQuote();
            foreach($cart->getAllAddresses() as $address){
                $address->unsetData('cached_items_nonnominal');
                $address->unsetData('cached_items_nominal');
            }
            $address = $quote->getShippingAddress();
            $zipcode = $address->getPostcode();
            $country = $address->getCountry();
            // Update the cart's quote.
            $ShipCart = Mage::getSingleton('checkout/cart');
            $shipAddress = $ShipCart->getQuote()->getShippingAddress();
            $shipAddress->setCountryId($country)->setPostcode($zipcode)->setCollectShippingrates(true);
            $ShipCart->save();
            $cart->setTotalsCollectedFlag(false);
            $cart->collectTotals();//echo 1 ;die;
            // Delegate to parent method to place an order for each seller
            parent::saveOrder();
            $lastOrderId = Mage::getSingleton('checkout/session')
                   ->getLastRealOrderId();
            if($allOrderId==''){
                $allOrderId=$allOrderId.$lastOrderId;
              }else{
                $allOrderId=$allOrderId.', '.$lastOrderId; 
            }
        }
        Mage::getSingleton('core/session')->setCurrentAllOrderId($allOrderId);
        return $this;
    }

But be aware that in Magento a payment is associated with an invoice, and each invoice is associated with an order.

In consequence this means that as soon as you have multiple orders, you will also have split the payments. So this is only feasible if the payment method doesn't require user interaction during the payment.

UPDATE: The orginal answer delegated to parent::save() which had to be parent:saveOrder(). It is fixed in the example code now.