Please see demo:
When you set .a and .b to inline-block elements, you can set their parent to text-align center. Then this is your requirement.
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>JS Bin</title>
<style>
.tc{text-align:center;}
.a, .b{display:inline-block; width:300px; *display:inline; *zoom:1;}
.a{background:blue;}
.b{background:red;}
</style>
</head>
<body>
<div class="tc">
<div class="a">aaa</div>
<div class="b">bbb</div>
</div>
</body>
</html>