Probably isn't the most convenient solution (not sure if there is a way to do this straight through .htaccess), but I would try this:
# .htaccess
php_value auto_prepend_file alter_host.php
# alter_host.php
<?php
$_SERVER['HTTP_HOST'] = 'anotherdomain.com';
?>
It caused some issues with me through the Laravel framework, but it worked with a simple site..so I'd give it a go.