Model within a model

I can't think of any reason why not, have you tried? 

To match your model names:

PHP Code:

<?php

namespace App\Models;

use CodeIgniter\Model;
use App\Models\Settings_model;
class Users_model extends Model {

    function __construct()
    {
        // Call the Model constructor
        parent::__construct();
        $this->Settings = new Settings_model();
    }
} 


PHP Code:

<?php

namespace App\Models;

use CodeIgniter\Model;
use App\Models\Users_model;
class Settings_model extends Model {

    function __construct()
    {
        // Call the Model constructor
        parent::__construct();
         $this->Users = new Users_model();
    }
}