How to combine conditions with several selector?

Hy, I'm trying to combine these condition:

$(window).scroll(function() {

if ($('#box2').isOnScreen() == true){
        $('#b2').css('opacity', '1')}
else{$('#b2').css('opacity', '0')};
        });

$(window).scroll(function() {

if ($('#box3').isOnScreen() == true){
        $('#b3').css('opacity', '1')}
else{$('#b3').css('opacity', '0')};
        });

I tried something like that:

$(window).scroll(function() {

    if ($('.box').isOnScreen() == true){
            $(this).css('opacity', '1')}
    else{$(this).css('opacity', '0')};
            });

but it don't work

my html look likes this

<div id="box2" class="box"></div>
<div id="box3" class="box"></div>

Thanks