Hi.. Please try this..

Using CodeBehind

protected void Button1_Click(object sender, EventArgs e)
{
this.Label1.Text = "Button1 clicked!";
}

protected void Button2_Click(object sender, EventArgs e)
{
somefunction();
}

private void somefunction()
{
string sender = "";
EventArgs e = new EventArgs();
this.Button1_Click(sender, e);
}

Using JavaScript

var mybutton = document.getElementById(yourButtonName);
if (mybutton)
{
mybutton.click();
}