title: Use of [(ngModel)] within FormGroup
body: <p>I have in my application a registration form. But within this registration form there is an optional 'password' and 'repeat password' input. Since I'd rather not use the <code>FormControl</code> object to retrieve these 2 values (other values are fine), I would like a workaround for the usage of <code>ngModel</code> within a <code>&lt;form&gt;</code></p>
  
  <hr>
  
  <h2>MCVE</h2>
  
  <p>TS: </p>
  
  <pre><code>public password: string = '';
  public passwordRe: string = '';
  public registrationForm;
  
  constructor(public fb: Formbuilder) {
      this.registrationForm = this.fb.group({
          'firstname' : [null, Validators.required],
          'lastname': [null, Validators.required]
      });
  }
  </code></pre>
  
  <p>HTML</p>
  
  <pre><code>&lt;form [formGroup]="registrationForm" (ngSubmit)="doSomething()"&gt;
      &lt;div class="form-group"
              [ngClass]="{'has-error':!registrationForm.controls['firstname'].valid 
                  &amp;&amp; registrationForm.controls['firstname'].touched}"&gt;
          &lt;label&gt;Firstname: *&lt;/label&gt;
          &lt;input class="form-control" type="text" placeholder="Firstname" [formControl]="registrationForm.controls['firstname']"/&gt;
      &lt;/div&gt;
  
      &lt;div class="form-group"
              [ngClass]="{'has-error':!registrationForm.controls['lastname'].valid 
                  &amp;&amp; registrationForm.controls['lastname'].touched}"&gt;
          &lt;label&gt;Lastname: *&lt;/label&gt;
          &lt;input class="form-control" type="text" placeholder="Lastname" [formControl]="registrationForm.controls['lastname']"/&gt;
      &lt;/div&gt;
  
      &lt;!-- NG MODELS --&gt;
  
      &lt;input type="password" [(ngModel)]="password"/&gt;
      &lt;input type="password" [(ngModel)]="passwordRe" /&gt;
  
      &lt;input type="submit" value="Some submit button"/&gt;
  &lt;/form&gt;
  </code></pre>
  
  <hr>
  
  <p>This page is shown on multiple pages as a child, via a selector. Placing the passwords on top, outside of the form would be the laziest solution, but I'd have to change some code to get that. (plus it doesn't look good) So I was wondering if there's a workaround for this specific issue.</p>