Two Ember Data Stores

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

models.comment.js

import Model from 'ember-data/model';
import attr from 'ember-data/attr';
import { belongsTo, hasMany } from 'ember-data/relationships';

export default Model.extend({
  text: attr()
});

models.post.js

import Model from 'ember-data/model';
import attr from 'ember-data/attr';
import { belongsTo, hasMany } from 'ember-data/relationships';

export default Model.extend({
  title: attr(),
  comment: belongsTo('comment', { async: false, inverse: null })
});

routes.application.js

import Ember from 'ember';
import { inject as service } from '@ember/service';

export default Ember.Route.extend({
  customStore: service(),
  
  model() {
    let store = this.store;
    let customStore = this.customStore;
    let comment = store.push({
      data: {
        id: "comment-1",
        type: "comment",
        attributes: {
          text: "My comment"
        }
      }
    });
    
    let post = customStore.createRecord('post', {
      title: 'My post'
    });
    
    post.set('comment', comment);
    
    return post;
  }
});

services.custom-store.js

import Ember from 'ember';
import DS from 'ember-data';

export default DS.Store.extend({
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
post: {{model.title}}

comment: {{model.comment.text}}
<br>
<br>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.1"
  }
}