using System;
using System.Windows.Forms;
 
namespace daniweb
{
  public partial class frm1 : Form
  {
    public frm1()
    {
      InitializeComponent();
    }
 
    public void EnableButton2()
    {
      button2.Enabled = true;
    }
 
    private void frm1_Load(object sender, EventArgs e)
    {
      button1.Enabled = true;
      button2.Enabled = false;
    }
 
    private void button1_Click(object sender, EventArgs e)
    {
      new frm2().Show();
      //this.Hide();
    }
 
    private void button2_Click(object sender, EventArgs e)
    {
      //Dont know what you want to do
    }
  }
}

using System;
using System.Windows.Forms;
 
namespace daniweb
{
  public partial class frm2 : Form
  {
    public frm2()
    {
      InitializeComponent();
    }
 
    private void button1_Click(object sender, EventArgs e)
    {
      frm1 f = GetMainForm();
      if (f == null)
        return;
      f.EnableButton2();
      //this.Hide();
      f.BringToFront();
      f.Focus();
    }
 
    private static frm1 GetMainForm()
    {
      for (int i1 = 0; i1 < Application.OpenForms.Count; i1++)
      {
        if (Application.OpenForms[i1] is frm1)
          return ((frm1)Application.OpenForms[i1]);
      }
      return null;
    }
 
  }
}