call one controller to another controller

You mix up controllers and libraries.
A library is a class with methods (functions) you can use in any controller where you load that library.

A controller always extends the CI_Controller class (or MY_Controller). That's why CI recognizes it as a controller.
The methods inside a controller are triggered by the url that you are using, or from within the controller itself.

Libraries must be located in the libraries folder.
The methods of the library are available after loading the library (in your controller).
More info: https://www.codeigniter.com/userguide3/g...aries.html