To forme

    public partial class Form1 : Form
    {
        Form2 Reftilandetvindue;
        public TextBox Textbox1 = new TextBox();

        public Form1()
        {
            InitializeComponent();
        }

        private void btnForm1_Click(object sender, EventArgs e)
        {
            Reftilandetvindue.Textbox2.Text = Reftilandetvindue.Textbox2.Text + "X";
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            Reftilandetvindue = new Form2(this);
            Reftilandetvindue.Show();
            Textbox1 = txtForm1;
        }
    }

	public partial class Form2 : Form
    {
        Form1 RefTilForm1;
        public TextBox Textbox2;

        public Form2(Form1 ReferenceTilKaldendeObjekt)
        {
            InitializeComponent();
            RefTilForm1 = ReferenceTilKaldendeObjekt;
        }

        private void btnForm2_Click(object sender, EventArgs e)
        {
            RefTilForm1.Textbox1.Text = RefTilForm1.Textbox1.Text + "X";
        }

        private void Form2_Load(object sender, EventArgs e)
        {
            Textbox2 = txtForm2;
        }
    }