You can rather do:
angular.module("myApp")
.controller(...)
for both the controllers, rather than writing the variable here
Note: app.js should be loaded before any of the controllers for this to work.
This is one of the ways it can be done:
<html ng-app="myApp">
<head>
<title></title>
</head>
<body>
<script type="text/javascript" src="app.js"></script>
<script type="text/javascript" src="controller1.js"></script>
<script type="text/javascript" src="controller2.js"></script>
</body>
</html>
Apart from this you can also use some task runners like gulp OR grunt to do that automatically.