$(document).ready(function(){
    $(".addMultipleTrendEvent").click(function () { 
        var counter = 0;
        $(".addMultipleTrendEvent > input").each( function () {
            if(this.checked == true) {
                counter++;
            }
        });
        
        if(counter > 1){
            $("#multipleNote").show();
        }
    });
});