How to override core class which is already overrided by other custom module in Magento 2

Module ONE is overriding some core class, if you want to override same core class already used/override module ONE, override Module ONE class by using your module( Module Two )as below example.

Module ONE as Vendor/Module

Vendor/Module is extending Magento\CatalogSearch\Block\Result

In di.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Magento\CatalogSearch\Block\Result" type="Vendor\Module\Block\Result"/>
</config>

In Block result.php

<?php
/**
 * Copyright © 2016 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Vendor\Module\Block;


/**
 * Product search result block
 */
class Result extends \Magento\CatalogSearch\Block\Result
{
   public function getSearchQueryText()
    {

die('Module ONE');
// Some code
return parent::getSearchQueryText();
    }

}

Module TWO as Vendor/Moduletwo

Vendor/Moduletwo is extending Vendor\Module\Block\Result

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Vendor\Module\Block\Result" type="Vendor\Moduletwo\Block\Result"/>
</config>

In block result.php

<?php
/**
 * Copyright © 2016 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Vendor\Moduletwo\Block;


/**
 * Product search result block
 */
class Result extends \Vendor\Module\Block\Result
{
   public function getSearchQueryText()
    {

die('TWO');
// Some code
return parent::getSearchQueryText();
    }

}

In this case both extension functionalities will work.