An improvement on my original gist that splits the WordPress navigation into two menus (for sites with a centered logo for example). This version accounts for sites that are also using dropdown menus. It will only take the top level menu items into account when calculating the breakpoint location.

mbt_split_nav_walker_with_child_nav.php

/* ========================================================================== 
    Custom Walker: Split menu based on item count
   ========================================================================== */

class mbt_split_nav_walker extends Walker_Nav_Menu {

    var $current_menu = null;
    var $break_point  = null;


    function start_el(&$output, $item, $depth, $args) {

        global $wp_query;

            // Get the locations of nav menus
            $theme_locations = get_nav_menu_locations();
         
            // Get the menu object of the current nav menu based on the returned theme location
            $menu_obj = get_term( $theme_locations[$args->theme_location], 'nav_menu' );



            // Get sub menu/child counts
            $menu_items = wp_get_nav_menu_items($menu_obj->term_id); 
            $menu_item_parents = array_map(function($o) { return $o->menu_item_parent; }, $menu_items);
            $child_count = array_count_values($menu_item_parents); 

            // Remove menus with no submenu form array
            unset($child_count[0]);
        
            // add remaining menu counts for total sub menu count
            $child_count = array_sum($child_count);

            // Get parent menu item sequential counts 
            $menu_counter = 0;
            
            global $menu_counter;            
            
            if($depth == 0) {
                $menu_counter++;
            }



            // set current menu object
            if( !isset( $this->current_menu ) ) {
                $this->current_menu = wp_get_nav_menu_object( $menu_obj->term_id );
            }
            
            // set breakpoint
            if( !isset( $this->break_point ) ) {

                $divisor = $this->current_menu->count - $child_count; 

                $this->break_point = ceil( $divisor / 2 ) + 1; 
            }  

            
          

        $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

        $class_names = $value = '';

        $classes = empty( $item->classes ) ? array() : (array) $item->classes;
        $classes[] = 'menu-item-' . $item->ID;

        $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
        $class_names = ' class="' . esc_attr( $class_names ) . '"';

        $id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
        $id = strlen( $id ) ? ' id="' . esc_attr( $id ) . '"' : '';

        if( ($this->break_point == $menu_counter) && ($depth == 0) ) {
            $output .= $indent . '</li></span></ul><ul class="nav top-nav cf right"><span><li' . $id . $value . $class_names .'>';
        } else {
            $output .= $indent . '<li' . $id . $value . $class_names . '>';
        }

        $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
        $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
        $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
        $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

        $item_output = $args->before;
        $item_output .= '<a'. $attributes .'>';
        $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;

        //$item_output .= $menu_counter;

        $item_output .= '</a>';
        $item_output .= $args->after;

        $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );

        
    }
}