több elem kiválasztása szinkronban

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
  public partial class Form1 : Form
  {
    public Form1()
    {
      InitializeComponent();
    }

    private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
      int darab = listBox1.Items.Count;
      for (int i = 0; i < darab; i++)
      {
        if(listBox1.GetSelected(i) && !listBox2.GetSelected(i))
        {
          listBox2.SetSelected(i, true);
        }
        else if(!listBox1.GetSelected(i) && listBox2.GetSelected(i))
        {
          listBox2.SetSelected(i, false);
        }
      }
      indexek(sender);
    }

    private void listBox2_SelectedIndexChanged(object sender, EventArgs e)
    {
      int darab = listBox2.Items.Count;
      for (int i = 0; i < darab; i++)
      {
        if (listBox2.GetSelected(i) && !listBox1.GetSelected(i))
        {
          listBox1.SetSelected(i, true);
        }
        else if (!listBox2.GetSelected(i) && listBox1.GetSelected(i))
        {
          listBox1.SetSelected(i, false);
        }
      }
      indexek(sender);
    }

    private void indexek(object sender)
    {
      ListBox lista = new ListBox();
      lista = (ListBox)sender;
      var valasztottak = lista.SelectedIndices;
      int valasztottdarab = lista.SelectedIndices.Count;
      richTextBox1.ResetText();
      for (int i = 0; i < valasztottdarab; i++)
      {
        richTextBox1.Text += (valasztottak[i] + 1).ToString();
        richTextBox1.Text += "\n";
      }
    }
  }
}