proxying solution

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  user: Ember.inject.service(),
  onControllerBool: true,
  computedFromPolicy: Ember.computed.alias('user.policy.canFind')
});

controllers.user.js

import Ember from 'ember';

export default Ember.Controller.extend({
  userThing: true,
  computedThing: Ember.computed('userThing', function(){
    return this.get('userThing');
  }),
});

mixins.proxy.js

import Em from 'ember';

export default Em.Mixin.create({
    unknownProperty: function(key) {
        return this.get('proxy').get(key);
    },
    init() {
        this._super(...arguments);
        var controller = this.get('proxy');
        for (var method in controller) {
            if (typeof controller[method] === 'function') {
                if (!this[method]) {
                    this.proxyMethod(method);
                }
            }
        }
    },
    proxyMethod: function(method) {
        this[method] = function() {
            var controller = this.get('proxy');
            return controller[method](...arguments);
        };
    },
});

services.user.js

import Ember from 'ember';
import ProxyMixin from '../mixins/proxy';

export default Ember.Controller.extend(ProxyMixin, {
  proxy: Ember.computed(function() {
        return this.container.lookup('controller:user');
   })
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>

<p>Both properties should change when clicking the checkbox</p>

user controller bool: {{input type="checkbox" checked=user.userThing}}
<br>
value of controller bool: {{user.userThing}}
<br>
value of computed off of controller bool: {{user.computedThing}}

<br>
<br>
{{outlet}}
<br>
<br>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "1.13.13",
    "ember-template-compiler": "1.13.13",
    "ember-testing": "1.13.13"
  },
  "addons": {
    "ember-data": "1.13.15"
  }
}