Well you actually could use an <iframe>
containing the second page in a floating <div>
-element. But as always, it's not always good just because it exists or because it was technically possible.
Except for the use of frame, you should never have more than one <html>
and never more than one <body>
It would be the best to have something like this:
<html>
<head><!-- header goes here --></head>
<body style="background-image: url('first.png');">
<img src="second.png"/>
</body>
</html>
Sure you could extract the css-part into a css-file. And you could set the style-attribute for another tag as well, it doesn't forcibly has to be <body>
hope that helps a bit.