How to place a function from another class in add_action 2nde argument?

The problem is that you’re trying to callback the process_action_heb method from $this (aka class A) and it’s not a method of the A class – it’s a method of B.

I’d use dependency injection to solve this, though you could use a method like you’re using.

//* plugin.php
include_once PATH_TO . '/classA.php';
include_once PATH_TO . '/classB.php';

$classB = new classB();
$classA = new classA( $classB );

add_action( 'hookA', [ $classA, 'some_method' ], 20 );

//* class-a.php
class A {
  protected $classB;
  public function __construct( $classB ) {
    $this->classB = $classB;
  }
  public function some_method() {
    //* Do something

    //* And add action to classB::some_method
    add_action( 'hookB', [ $this->classB, 'some_method' ] );
  }
}

//* class-b.php
class B {
  public function some_method() {
    //* This code will run on the hookB hook
  }
}