// No "sender" or event args
public void button2_click(object sender, EventArgs e)
{
button1_click(null, null);
}
или
// Button2's the sender and event args
public void button2_click(object sender, EventArgs e)
{
button1_click(sender, e);
}
или, как указал Джоэл:
// Button1's the sender and Button2's event args
public void button2_click(object sender, EventArgs e)
{
button1_click(this.button1, e);
}