function debounce (func, wait, immediate) {
    var timeout;
    return function () {
        var context = this, args = arguments;
        var later = function () {
            timeout = null;
            if (!immediate) func.apply(context, args);
        };
        var callNow = immediate && !timeout;
        clearTimeout(timeout);
        timeout = setTimeout(later, wait);
        if (callNow) func.apply(context, args);
    };
};

function _syncScroll (src, dst) {
    var ratio = dst.scrollHeight / src.scrollHeight;
    dst.scrollTop = Math.round(src.scrollTop * ratio);
}

var syncScroll = debounce(_syncScroll, 5);

var one = document.getElementById('one');
var two = document.getElementById('two');

one.addEventListener('scroll', syncScroll.bind(null, one, two));
two.addEventListener('scroll', syncScroll.bind(null, two, one));