Backbone.Model.fetch does not support the silencing of sync events. Further, ES5 lacks generator expressions, as such semaphore implementations require some hacks. Semaphores are useful in this context as populating a model with data from multiple sources is a model operation.

BackboneSyncSempahores

var Yolo = Backbone.Model.extend({

  sweetMethods : function...

  ...

  sem : {
    requiredSyncs : 2,
    syncedSyncs : 0
  },

  fetch : function( options ){
  
    options = options ? _.clone(options) : {};
    if (options.parse === void 0) options.parse = true;
    var model = this;
    var success = options.success;
    options.success = function(resp) {
    if (!model.set(model.parse(resp, options), options)) return false;
    if (success) success(model, resp, options);
      model.sem.syncedSyncs += 1;

      if( model.sem.syncSyncs >= model.sem.requiredSyncs ){
    
       this.trigger( 'sync', model, resp, options );

      }
    };
    wrapError(this, options);
       

   return this.sync('read', this, options);

  }

}
});

var yolo = new Yolo();

yolo.on( 'sync', function(){
  console.log( 'yo I've been fetched twice naow.' );
});

yolo.fetch();