pew.js

(function() {
    var shouldCall = false;

    function first() {
        console.log("first callback")
        if (!shouldCall) {
            shouldCall = true;
        } else {
            firstFinal();
        }
    }

    function second() {
        console.log("second callback")
        if (!shouldCall) {
            shouldCall = true;
        } else {
            firstFinal();
        }
    }

    function firstFinal() {
        console.log("first finished");
    }

    setTimeout(first, 500);

    setTimeout(second, 1000);
})();

// output:
// first callback
// second callback
// first finished