If you have installed the Twig and the parser package, you only need to specify the extension.
View::forge('viewname'); // will load the viewname.php view file if view_ext was set to '.php' View::forge('viewname.twig'); // will load the viewname.twig view file through the Twig engine
So the config only defines the default, for when you use view names without specifying the extension.
If you use multiple template systems at the same time (like both php and twig templates), you need to decide with one will be your default, and you have to hardcode the type for the other.
You can not dynamically switch from one type to the other on a per-file basis, without changing your code.