One library depends on another - what's the solution?

Hello,

To make sure I use the proper practice, I have to fully explain what's going on.

I have a library called 'Ui', responsible for displaying the front end. (It displays a main view that has other views inside it, variables for each view, and so on.)
The object for this library is created in BaseController::initController(). So all controllers have access to the $this->ui object. I suppose this is fine so far.

I have another library however, called 'Auth', which is responsible for handling the user logged in. Both classes are autoloaded like this:

PHP Code:

public $classmap = [
  'Ui' => APPPATH.'/Libraries/Ui.php',
  'Auth' => APPPATH.'/Libraries/Auth.php',
]; 


The constructor of Auth has to set a variable for Ui, which means Ui depends on Auth*. The two libraries have no access to one another though.
* EDIT: Auth depends on Ui

This same method worked fine in CI 2-3, where Auth simply executed a $this->ui->... method.

Is there a simple solution for this, or is the whole concept wrong? Should I use libraries for both these things in the first place?

Thanks for your time.