How to compare two cells value in Grid via Compare Validator?

protected void RadGrid1_ItemCreated(object sender, GridItemEventArgs e)
{
if (e.Item is GridEditableItem && e.Item.IsInEditMode)
{
GridEditableItem item = e.Item as GridEditableItem;
RadTimePicker start = (RadTimePicker)item.FindControl("rtpStartTime");
RadTimePicker end = (RadTimePicker)item.FindControl("rtpEndTime");

TableCell cell = (TableCell)start.Parent;

CompareValidator val = new CompareValidator();
val.ControlToCompare = start.ID;
val.ControlToValidate = end.ID;
val.Operator = ValidationCompareOperator.GreaterThan;
val.ErrorMessage = "Error message";
cell.Controls.Add(val);
}
}