Yes, it is possible to float a Tile on top of another Tile. Use the BasicLayout to do this and specify (x,y) or top,left positioning with the Tile.
HOWEVER: this will NOT work with HTML Tiles when using the Web version. HTML Tiles are placed into IFRAMEs that are floating above the Flash Player and so will obscure any Tiles positioned “over” them. So as long as you are using SWF-based Tiles, you should not have a problem.