myRoute.js

import Route from '@ember/routing/route';

export default Route.extend({
    model() {
        return this.store.findMultiple(['tumblr-post-photo', 'tumblr-post-text']);                
      }
});

store.js

import DS from 'ember-data';
import { all } from 'rsvp';
import { A } from '@ember/array';

export default DS.Store.extend({
    findMultiple: function (types) {
      var self = this;
      var recordsByType = types.map(function (type) {
        return self.findAll(type);
      });
      return self.mergeArrayPromises(recordsByType);
    },
  
    mergeArrayPromises: function (promises) {
      var promise = all(promises).then(function(arrays) {
        var mergedArray = A();
        arrays.forEach(function (records) {
          mergedArray.pushObjects(records.toArray());
        });
        return mergedArray;
      });
  
      return DS.PromiseArray.create({
        promise: promise,
      });
    },
  });