Okay, so you are trying to update 2 "sibling" grids. That's another story. Unfortunately at this time, a grid that initiates a callback can only update within itself. ASP.NET provides basically no support for more advanced updating within callbacks (UpdatePanels actually use postbacks, not callbacks, and are very heavy things). We are in the process of improving this, and in upcoming releases of our products you will be able to update many separate controls during a callback. Until then, you are best off just using a regular <asp:Button and handling its click event. This will use a postback and although the flickering will occur, your problem should be solved.
You may need to call DataBind on the second grid, because it has no official way to know that its data depends on another.
Hope this helps!