C#

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 WindowsFormsApplication13
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void label2_Click(object sender, EventArgs e)
        {

        }

        Boolean bB2ShouldStop = false;
        Boolean bB1ShouldStop = false;
        private void button1_Click(object sender, EventArgs e)
        {
            if (label1.Text == "500")
            {
                for (int i = 1; (i <= 500) && !bB1ShouldStop; i++)
                {
                    label1.Text = i.ToString();
                    System.Threading.Thread.Sleep(10);
                    Application.DoEvents();
                    if (label2.Text != "500")
                    {
                        bB2ShouldStop = true;
                        label2.Text = (Int16.Parse(label2.Text) + 1).ToString();
                    }

                }
                bB1ShouldStop = false;    
            }            
        }

        private void button2_Click(object sender, EventArgs e)
        {
            if (label2.Text == "500")
            {
                for (int i = 1; (i <= 500) && !bB2ShouldStop; i++)
                {
                    label2.Text = i.ToString();
                    System.Threading.Thread.Sleep(10);
                    Application.DoEvents();
                    if (label1.Text != "500")
                    {
                        bB1ShouldStop = true;
                        label1.Text = (Int16.Parse(label1.Text) + 1).ToString();
                    }

                }
                bB2ShouldStop = false; 
            }            
        }

        
    }
}