Hi LapD,
Simply use the below code inside your header section:
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js" ></script>
<script>
jQuery(function(){
var max = 2;
var checkboxes = $('input[type="checkbox"]');
checkboxes.change(function(){
var current = checkboxes.filter(':checked').length;
checkboxes.filter(':not(:checked)').prop('disabled', current >= max);
});
});
</script>