create instance of two classes together

Answer 1

You can instantiate dozens/hundreds/X of objects of any kind within the same php instance.
Use a debugger or add more debug (echo) code to find the error.

e.g. using this dummy implementation of the two classes

class CategoryList {
  public $mCategory=null;
  public function init() {
    $this->mCategory = array(
      array('name'=>'Cat A'),
      array('name'=>'Cat B'),
    );
  }
}

class BrandList {
  public $mBrand=null;
  public function init() {
    $this->mBrand = array(
      array('name'=>'Brand A'),
      array('name'=>'Brand B'),
    );
  }
}

your code prints

<a href=''>Cat A<br/></a><a href=''>Cat B<br/></a><a href=''>Brand A<br/></a><a href=''>Brand B<br/></a>

without any problem.

Answer 2

No, you can create any number of instances even for the same class. Make sure that your both classes are included in your script independently of each other.