Behat mink ChromeDriver multiple independent sessions are not stopping independently

Question

I'm trying to have multiple sessions of Behat Mink ChromeDriver running in the same time independently.

First of all I start Chrome with the right debugging port:

$ google-chrome-stable --remote-debugging-address=0.0.0.0 --remote-debugging-port=9222

Let's say I want two sessions that I start like this:

use Behat\Mink\Mink;
use Behat\Mink\Session;
use DMore\ChromeDriver\ChromeDriver;

// session 1
$driver1 = new ChromeDriver('http://localhost:9222', null, 'http://www.google.com');
$mink1 = new Mink(array('tab1' => new Session($driver1)));
$mink1->setDefaultSessionName('tab1');
$mink1->getSession()->visit("http://www.google.com");

// session 2
$driver2 = new ChromeDriver('http://localhost:9222', null, 'http://www.google.com');
$mink2 = new Mink(array('tab2' => new Session($driver2)));
$mink2->setDefaultSessionName('tab2');
$mink2->getSession()->visit("http://www.bing.com");

//changing the url of session 1
$mink1->getSession()->visit("http://www.yahoo.com");

Everything goes smoothly until here: Chrome is open and I have two tabs with the correct url displayed. I can change the url of each session independently, everything is all right. But when I stop one of the two sessions, the two sessions stop together:

$mink2->getSession()->stop();

The two tabs are closed and the session 1 ($mink1) is no longer accessible:

$mink1->getSession()->visit("http://www.bing.com");    
DMore/ChromeDriver/StreamReadException

Any idea why this is happening? In the documentation (http://mink.behat.org/en/latest/guides/managing-sessions.html) it says that each session must use a different driver instance. I believe it's what I'm doing, right?