static class Class1
    {
        public static double[] MyArray = new double[100];

        public static void MyArrayChange()
        {
            foreach (Form f in Application.OpenForms)
            {
                if (f is Form2)
                {
                    ((Form2)f).Form2Change();
                }
                if (f is Form3)
                {
                    ((Form3)f).Form3Change();
                }
            }
           
        }
    }

public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();

            this.textBox1.Text = Class1.MyArray[1].ToString();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Class1.MyArray[1] = Double.Parse(this.textBox1.Text);

            Class1.MyArrayChange();
        }

        public  void Form2Change()
        {
            this.textBox1.Text = Class1.MyArray[1].ToString();
        }
    }

public partial class Form3 : Form
    {
        public Form3()
        {
            InitializeComponent();

            this.textBox1.Text = Class1.MyArray[1].ToString();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Class1.MyArray[1] = Double.Parse(this.textBox1.Text);

            Class1.MyArrayChange();
        }


        public void Form3Change()
        {
            this.textBox1.Text = Class1.MyArray[1].ToString();
        }
    }