getting started with wrapping two stores with one

wrapperStore

var store = function(substores,options){

    return {
         query: function(query, options){
             return when(promise.all(substores.map(function (store) {
                 return when(store.query(query, options));
             }), function (results) {
                 return results.reduce(function (out, arr) {
                     return out.concat(arr);
                 }, []);
             });
         }
    }

}