Update Your Butshow_Click event as Mentioned Below :
protected void Butshow_Click(object sender, EventArgs e)
{
DateTime datefrom = DateTime.Parse(Textfrom.Text);
DateTime dateto = DateTime.Parse(Textto.Text);
inp.plandatefrom = datefrom;
inp.plandateto = dateto;
dt = inp.gridshow();
GridView1.DataSource = dt;
GridView1.DataBind();
DataTable grid = new DataTable();
foreach (DataRow row in dt.Rows)
{
DataTable dtf = new DataTable();
inf.plandat = DateTime.Parse(row[0].ToString());
grid = inf.gridshowfor();
inf.plandat = DateTime.Parse(row[0].ToString());
dtf = inf.gridshowfor();
if (grid.Rows.Count == 0)
{
grid = dtf;
}
else
{
foreach (var rowToAdd in dtf.Rows)
{
grid.Rows.Add(rowToAdd);
}
}
}
GridView2.DataSource = grid;
GridView2.DataBind();
}