Código CSS :
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
#contenedor{
width: 70%;
margin: auto;
background-color: red;
height: 800px;
position: relative;
}
#caja1{
width: 40%;
background-color: yellow;
height: 200px;
margin-left: 5%;
}
#caja2{
width: 40%;
background-color: blue;
height: 200px;
display: inline-block;
}
</style>
</head>
<body>
<div id="contenedor">
Utilización Display inline-block
<div id="caja1"></div>
<div id="caja2"></div>
</div>
</body>
</html> 