hi satej...
For preventing Refreshing the whole page,you need UpdatePanel for each grid, add updatepanel for two grids separately.
you try like this
first add script manager to your sourse code.
<asp:ScriptManager
ID="ScriptManager1" runat="server">
</asp:ScriptManager>
then add update panel like this
1st grid
<asp:updatepanel id="updateGrid1" runat="server" updatemode="Conditional" >
<contenttemplate>
//place your grid1 hear
</contenttemplate>
</asp:updatepanel>
2nd grid
<asp:updatepanel id="updateGrid2" runat="server" updatemode="Conditional" >
<contenttemplate>
//place your grid2 hear
</contenttemplate>
</asp:updatepanel>
All the Best
Thanks&&Regards
Sandeep