Find UserControl2 in Parent page and then Find Label control inside UserControl2 .
Here is the solution. Will post robust solution asap.
protected void Button1_Click(object sender, EventArgs e)
{
UserControl uc = base.Parent.FindControl("uc2") as UserControl;
if (uc != null)
{
uc.FindControl("lblUserControl2") as Label).Text = "its from usercontrol1 Button click event ";
}
}