JavaScript mixins - extending and super methods
The main things is that your system is much more verbose and harder to understand than simply hard-coding all the calls to superclass methods:

function A(a) {
    this.a = a;
}
A.prototype = {
    methodA: function () {
        console.log("a: "+this.a);
    }
};

function B(a) {
    this.b = a+1;
}
B.prototype = {
    methodB: function () {
        console.log("b: "+this.b);
    }
};

function Obj(a) {
    A.call(this, a);
    B.call(this, a);
    this.z = a-2;
}
// Using underscore.js extend() to mix in methods from A and B
Obj.prototype = _({
    methodZ: function () {
        console.log("z: "+this.z);
    }
}).extend(A.prototype, B.prototype);

var o = new Obj(7);
o.methodA();
o.methodB();
o.methodZ();


Instead you should have some sort of factory method that creates new classes for you, setting up the mixins behind the scenes. For example:

var Foo = createClass({
    mixins: [ClassA, ClassB],
    someMethod: function() { ... }
});


I suggest you study some existing libraries that implement their own JavaScript class systems. There are lots of nuances of the JS object model that you need to be aware of. I strongly suggest you don't use your own home-grown class-system for production, but instead adopt some already bettle-tested implementation. But writing you own is a good way to learn more deeply about JavaScript.