As Joes said, you can't give one object two backgrounds. But there's a very easy workaround for this:
Code:
<html>
<head>
<style>
body { background: url(...) ...; }
#outer { background: url(...) ...; height: 100%; width: 100%; }
</style>
<body>
<div id="outer">
... [ content ] ...
</div>
</body>
</html>