The highcharts supports only single legend. If you need additioanl, you should create it as html object, which will call a show/hide functionon series like in the example:
$legend = $('#customLegend');
$.each(chart.series[0].data, function (j, data) {
$legend.append('<div class="item"><div class="symbol" style="background-color:'+data.color+'"></div><div class="serieName" id="">' + data.name + '</div></div>');
});
$('#customLegend .item').click(function(){
var inx = $(this).index(),
point = chart.series[0].data[inx];
if(point.visible)
point.setVisible(false);
else
point.setVisible(true);
});