Вспоминаем про псевдо-элементы:
<html>
<head>
<style type="text/css">
.bg {
position: relative;
background-image: url("http://bonurtek.com/wp-content/uploads/2014/05/gradient-wallpaper-1.jpg");
background-size: cover;
margin: 0;
}
.setka:before {
content: '';
position: absolute;
width: 100%;
height: 100%;
background-image: url(http://r-g.esy.es/templates/web/images/noise.png);
background-size: 2px 2px;
z-index: -1;
}
</style>
</head>
<body class="bg setka">
<div>
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</div>
</body>
</html>