Creas 2 textbox vacios y un button con la propiedad enable = false.
Despues solo agregas esto:
private void textBox1_TextChanged(object sender, EventArgs e)
{
if ((textBox1.Text != "") && (textBox2.Text != "")) button1.Enabled = true;
else button1.Enabled = false;
}
private void textBox2_TextChanged(object sender, EventArgs e)
{
if ((textBox1.Text != "") && (textBox2.Text != "")) button1.Enabled = true;
else button1.Enabled = false;
}