Edit in JSFiddle

if($$('.abc').length == 2){
    $$('p').addClass('fail');
  $('change').set('html','but it selects <strong>two</strong> divs');  
} else {
    $$('p').addClass('success');
  $('change').set('html','and it works properly'); 
}

<div class="abc">ABC</div>
<div class="abcd">ABCD</div>
<p>Mootools selector should select <strong>one</strong> div <span id="change"></span></p>