Form submit to page with another Form
Use this code, your missing $return $form in the second form


    /**
     * Implements hook_menu().
     */
    function listings_menu() {
        $items = array();
        $items['site/listings'] = array(
            'title' => 'State Listings',
            'page callback' => 'drupal_get_form',
            'page arguments' => array('listings_form'),
            'access arguments' => array('access content'),
        );

        $items['site/listings/%/%'] = array(
            'title' => 'Business Listings',
            'page callback' => 'drupal_get_form',
            'page arguments' => array('listings_mini_form'),
            'access arguments' => array('access content')
        );

        return $items;
    }

    /*
     *  FIRST FORM
     */

    function listings_form($form, &$form_state) {

        $form['zipcode'] = array(
            '#title' => t('Zipcode'),
            '#type' => 'textfield',
            '#maxlength' => 5,
        );

        $form['form-wrapper']['form-wrapper-zip']['submit'] = array(
            '#type' => 'submit',
            '#value' => t('Submit')
        );

        return $form;
    }

    /*
     *  FIRST FORM SUBMIT FUNCTION
     */

    function listings_form_submit($form, &$form_state) {

        $zipcode = $form_state['values']['zipcode'];

        $form_state['redirect'] = 'site/listings/' . strtolower(listings_db_state($zipcode)) . '/' . strtolower(str_replace(' ', '+', listing_db_city($zipcode)));
    }

    /*
     *  2ND FORM ON PAGE IM CALLING
     */

    function listings_mini_form($form, &$form_state) {

        $form['mini-form'] = array(
            '#title' => t('Enter your Zip Code for more listings.'),
            '#type' => 'textfield',
            '#maxlength' => 5,
        );

        $form['submit'] = array(
            '#type' => 'submit',
            '#value' => t('Submit')
        );

        if (isset($form_state['values'])) {
            $form['page_output'] = array(
                '#markup' => listings_list_view($form, $form_state)
            );
        }
        return $form;
    }

    /*
     *  2ND FORM SUBMIT FUNCTION
     */

    function listings_mini_form_submit($form, &$form_state) {
        $zipcode = $form_state['values']['zipcode'];
        $form_state['redirect'] = 'site/listings/' . strtolower(listings_db_state($zipcode)) . '/' . strtolower(str_replace(' ', '+', listing_db_city($zipcode)));
    }

    /*
     *  FUNCTION WITH LISTINGS INSIDE OF IT 
     */

    function listings_list_view($form, &$form_state) {
        global $base_url;
        $zipcode = isset($form_state['values']['zipcode']);
        dpr($zipcode);
        #blah blah blah    
    }


I got output using this code


    /*
     *  FIRST FORM
     */

    function listings_form($form, &$form_state) {
        $form['zipcode'] = array(
            '#title' => t('Zipcode'),
            '#type' => 'textfield',
            '#maxlength' => 5,
        );

        $form['form-wrapper']['form-wrapper-zip']['submit'] = array(
            '#type' => 'submit',
            '#value' => t('Submit')
        );

        return $form;
    }

    /*
     *  FIRST FORM SUBMIT FUNCTION
     */

    function listings_form_submit($form, &$form_state) {
        $zipcode = $form_state['values']['zipcode'];
        $form_state['redirect'] = 'site/listings/' . $zipcode . '/' . $zipcode;
    }

    /*
     *  2ND FORM ON PAGE IM CALLING
     */

    function listings_mini_form($form, &$form_state) {
        $form = array();
        $form['mini-form'] = array(
            '#title' => t('Enter your Zip Code for more listings.'),
            '#type' => 'textfield',
            '#maxlength' => 5,
        );

        $form['submit'] = array(
            '#type' => 'submit',
            '#value' => t('Submit')
        );

        if (isset($form_state['values'])) {
            $form['page_output'] = array(
                '#markup' => listings_list_view($form, $form_state)
            );
        }
        return $form;
    }

    /*
     *  2ND FORM SUBMIT FUNCTION
     */

    function listings_mini_form_submit($form, &$form_state) {
        $zipcode = $form_state['values']['zipcode'];
        $form_state['redirect'] = 'site/listings/' . strtolower(listings_db_state($zipcode)) . '/' . strtolower(str_replace(' ', '+', listing_db_city($zipcode)));
    }

    /*
     *  FUNCTION WITH LISTINGS INSIDE OF IT 
     */

    function listings_list_view($form, &$form_state) {
        $zipcode = isset($form_state['values']['zipcode']);
    }