U want to edit on any button click (which is in main grid view) then here you have answer ,
protected void btnAddGrid_Click(object sender, EventArgs e)
{
Button btnAddGrid= (Button)sender;
GridViewRow grdMainRow = (GridViewRow)btnAddGrid.NamingContainer;
GridView grdSecond = (GridView)grdMainRow .FindControl("grdSecond ");
}
If your answer is not this , then let me.