function Equalize() {
  box1height = document.getElementById('box1').offsetHeight;
  box2height = document.getElementById('box2').offsetHeight; 
  if (box1height > box2height) 
    {document.getElementById('box2').style.height = box1height+"px";}
  else 
    {document.getElementById('box1').style.height = box2height+"px";}
}

Equalize();