how to copy one textbox value to another textbox in asp.net gridview using javascript

I am sending my coding also.i am not able wirte the javascript also.please help me any one.
<script type="text/javascript">
function OneTextToother()
 { var first=document.getElementById('<%=txtQuantity.ClientID %>').value;
document.getElementById('<%=txtQuantity1.ClientID %>').value=first; }
</script>
<asp:GridView ID="gvOrder" runat="server" OnRowDataBound="gvOrder_RowDataBound">        
<Columns>            
<asp:TemplateField>                
 <ItemTemplate>                    
<asp:TextBox ID="txtQuantity" runat="server" onkeyup="javascript:OneTextToother();"  ></asp:TextBo ...