You can't, unless it is set as a multiline textbox (Set it's Multiline property to "true"). Then, it's just a case of adding a newline to the text:
string a = textBox1.Text;
textBox2.Text += "\r\n" + a;
textBox1.Clear();
Or better:
textBox2.Text = string.Format("{0}\r\n{1}", textBox2.Text, textBox1.Text);
textBox1.Clear();
Since it generates fewer intermediate strings.