public override int SelectedIndex
{
      get
      {
            ...
      }
      set
      {
            int num1 = (this.itemsCollection == null) ? 0 : this.itemsCollection.Count;
            if ((value < -1) || (value >= num1))
            {
                  object[] objArray1 = new object[2] { "value", value.ToString() } ;
                  throw new ArgumentOutOfRangeException(SR.GetString("InvalidArgument", objArray1));
            }
            ......
            if ((this.selectionMode == SelectionMode.One) && (value != -1))
            {
                  .......
            }
            else if (value == -1)
            {
                  if (this.SelectedIndex == -1)
                  {
                        return;
                  }
                  this.ClearSelected();
            }
            else
            {
                  ....
            }
      }

 Private Sub ListBox1_GotFocus(ByVal sender As Object, ByVal e As System.EventArgs) Handles ListBox1.GotFocus, ListBox2.GotFocus
        If sender Is ListBox1 Then
            ListBox2.ClearSelected()
        Else
            ListBox1.ClearSelected()
        End If

    End Sub

this.listBox1.GotFocus +=new EventHandler(listBox1_GotFocus);
this.listBox2.GotFocus +=new EventHandler(listBox2_GotFocus);

private void listBox1_GotFocus(object sender, EventArgs e)
		{
			this.listBox2.ClearSelected();
		}

		private void listBox2_GotFocus(object sender, EventArgs e)
		{
			this.listBox1.ClearSelected();
		}