How To modify text..

   StringBuilder r = new StringBuilder();

            int i;

            for (i = 0; i < textBox1.Text.Length; i++)

            {

                r.Append(textBox1.Text.Substring(i, 1));

                if (textBox2.Text.Length > i)

                {

                    r.Append(textBox2.Text.Substring(i, 1));

                }

            }

            if (textBox2.Text.Length > i)

                r.Append(textBox2.Text.Substring(i));

            label2.Text = r.ToString();

You can do this (rather messily) with code such as this:

        string text1 = textBox1.Text;  // "josop"
        string text2 = textBox2.Text;  // "9970345678"
        char[] chars = new char[text1.Length + text2.Length];
        chars[0] = text1[0];
        chars[1] = text2[0];
        chars[2] = text1[1];
        chars[3] = text2[1];
        chars[4] = text1[2];
        chars[5] = text2[2];
        chars[6] = text1[3];
        chars[7] = text2[3];
        chars[8] = text2[4]; // is this the right way around?
        chars[9] = text1[4]; // ditto
        for(int i = 10; i < chars.Length; i++)
        {
            chars[i] = text2[i - 5];
        }
        string output = new string(chars);
        string message = String.Format("The combination of '{0}' and '{1}' is '{2}'", text1, text2, output);
        MessageBox.Show(message);

However, if you want to do this generally, you'll need to be clear on how many elements of each string are to be interleaved.