private void richTextBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    if (e.KeyChar == (char)Keys.Enter)
    {
        //This is were the code should be
    }
}

if (e.KeyChar == (char)Keys.Enter)
{
     richTextBox1.AppendText("t");

      // Edited After Comment
     var PrevLine = richTextBox2.Lines[richTextBox2.Lines.Count() - 1].ToString();
     var TabsCount = System.Text.RegularExpressions.Regex.Matches(PrevLine, "t").Count;
}

if (e.KeyChar == (char)Keys.Enter)
{
    richTextBox1.AppendText(Environment.NewLine);
    int prevLine = richTextBox1.GetLineFromCharIndex(richTextBox1.SelectionStart) - 1;
    MatchCollection mc = Regex.Matches(richTextBox2.Lines[prevLine], "s+");
    if (mc.Count > 0)
        richTextBox1.AppendText(mc[0].Value);
    e.Handled = true;
}