You need to add a LineBreak <br />
between the two texts:
You can do this with a Literal-Control:
oportunities.Controls.Add(box3);
oportunities.Controls.Add(new LiteralControl("<br/>"));
oportunidades.Controls.Add(box4);
Or by simply appending <br />
to the first label:
box3.Text = TextBox2.Text + "<br />";
oportunities.Controls.Add(box3);
oportunidades.Controls.Add(box4);