[Dilettante_Pro]

<style>
.green { background-color:green; }
.active { border:1px solid red;}
</style>
<button class="fn" >A</button>
<button id="w">B</button>
<button class="key">C</button>
<script src="https://code.jquery.com/jquery-latest.min.js"></script>
<script>
var     a = $(".fn"),
          b = $("#w"),
          k = $(".key");
a.click(function() {
    a.toggleClass("green active");
    testStatus();
});
b.click(function() {
    b.toggleClass("green active");
    testStatus();
});
function testStatus() {
  if (a.hasClass('active') && b.hasClass('active')) {
     k.addClass('green');
  } else { k.removeClass('green'); }

}
</script>