Magento : sharing the cart between stores

The first thing enable the Use SID on Frontend at
System→Configuration→Web→Session

Then replace following code in start() method of Mage_Core_Model_Session_Abstract_Varien
if (isset($_SESSION)) {
            return $this;
        }

with
if (isset($_SESSION) && !isset($_GET['SID'])) {
            return $this;
        }