image_lib... how to scale image down and then create a thumb in one function?

Code:
// resize original to smaller 500 500

            $resize_config['image_library'] = 'GD2';

            $resize_config['source_image'] = './images/photos/' . $photo_info['file_name'];

            $resize_config['maintain_ratio'] = TRUE;            

            $resize_config['create_thumb'] = FALSE;

            $resize_config['width'] = 500;

            $resize_config['height'] = 500;

            $this->load->library('image_lib', $resize_config);

            $this->image_lib->resize();                

            

            // create thumb

            $thumb_config['image_library'] = 'GD2';

            $thumb_config['source_image'] = './images/photos/' . $photo_info['file_name'];

            $thumb_config['maintain_ratio'] = TRUE;

            $thumb_config['new_image'] = './images/photos/t_' . $photo_info['file_name'];

            $thumb_config['thumb_marker'] = '';

            $thumb_config['create_thumb'] = TRUE;            

            $thumb_config['width'] = 150;

            $thumb_config['height'] = 150;

            $this->load->library('image_lib', $thumb_config);

            $this->image_lib->resize();