<script type="text/javascript"> 
   
    var map;
    var geoXml; 
    var toggleState =  1 ;

    function initialize() {
      if (GBrowserIsCompatible()) {
        geoXml = new GGeoXml("http://net/1.kml","http://net/2.kml");
        map = new GMap2(document.getElementById("map_canvas")); 
        map.setCenter(new GLatLng( 50 . 60 ,  26 . 25 ),  11 ); 
        map.setUIToDefault();
        map.addOverlay(new GGeoXml("http://net/1.kml")); 
        map.addOverlay(new GGeoXml("http://net/2.kml"));
      }
    } 

    function toggleMyKml() {
      if (toggleState ==  1 ) {
        map.removeOverlay(geoXml);
        toggleState =  0 ;
      } else {
        map.addOverlay(geoXml);
        toggleState =  1 ;
      }
    }
    </script>
.......................................
<input type="button" value="Toggle KML" onClick="toggleMyKml();"/>