edulan

const takePhoto = () => new Promise((resolve) => (setTimeout(resolve, Math.random() * 1E3 + 5E2, 'photo')))

const makeCountdown = (seconds) =>
	Rx.Observable
    .concat(
      Rx.Observable
        .interval(1E3)
        .take(seconds),
      Rx.Observable.defer(() => takePhoto())
    )

javisantana

 var f = _.after(2, function() {
   // do something with both collections               
 });
 this.collection1.bind('reset', f);
 this.collection2.bind('reset', f);