Can plugin have two links in admin navigation? Is it possible to add another link to admin navigation? For example I have plugin clients which has it's own Button in the admin menu, in the same plugin i have newsletter so I would like to have that Newsletter in that menu as well.
The top links in the CP header are often called nav tabs.

Beyond the "default" nav tab for your plugin (pointing to the plugin's CP section), it's possible to declare additional tabs via the modifyCpNav hook (added in Craft 2.3.2640):

public function modifyCpNav(&amp;$nav)
{
    $nav['mypluginhandle-newsletter'] = array('label' =&gt; 'Newsletter', 'url' =&gt; 'mypluginhandle/newsletter');
}


Of course, this will only create a tab with a link pointing to whatever you put in the url property. If you haven't already created the page for the newsletter, you'll need to do so by creating a route to a template or controller:

public function registerCpRoutes()
{
    return array(
        'mypluginhandle/newsletter' =&gt; 'myPluginHandle/newsletter',
    );
}


Read more about the registerCpRoutes hook in the official docs.

Both the modifyCpNav() and registerCpRoutes() methods should go in your plugin's main class.