disconnectOutlet

controllers.application.js

import Ember from 'ember';

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

controllers.modal1.js

import Ember from 'ember';

export default Ember.Controller.extend({
  actions:{
   close(){
    this.send('closeModal'); 
   }
  }
});

controllers.modal2.js

import Ember from 'ember';

export default Ember.Controller.extend({
  actions:{
   close(){
    this.send('closeModal'); 
   }
  }
});

routes.application.js

import Ember from 'ember';

export default Ember.Route.extend({
  actions: {
    openModal: function(modalName) {
      this.get('controller').set('modalOpen', true);
      return this.render(modalName, {
        into: 'application',
        outlet: 'modal'
      });
    },
    closeModal(){
      this.get('controller').set('modalOpen', false);
      this.disconnectOutlet({
      	outlet: 'modal',
        parentView: 'application'
      });
    }
  }
});

styles.app.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
  position: relative;
}

.modal{
  background: rgba(33, 33, 33, .7);
}
.modal-open{ 
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
}

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<h2>modalOpen: {{modalOpen}}</h2>
<br>
<br>
{{outlet}}
<br>
<br>

<div class="modal {{if modalOpen 'modal-open'}}">{{outlet 'modal'}}</div>

<button {{action 'openModal' 'modal1'}}>open modal 1</button>
<button {{action 'openModal' 'modal2'}}>open modal 2</button>

templates.modal1.hbs

<h2>modal1</h2>

<button {{action 'close'}}>close</button>

templates.modal2.hbs

<h2>modal2</h2>

<button {{action 'close'}}>close</button>

twiddle.json

{
  "version": "0.9.3",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.6.0",
    "ember-data": "2.6.1",
    "ember-template-compiler": "2.6.0"
  }
}