example.php

<?php
$formset = new FormSet();
$formset->addForm(
    $this->createForm('form_1', $foo),
    function($form) use ($commandBus) {
        $commandBus->execute($form->getData());
    }
);
$formset->addForm(
    $this->createForm('form_1', $bar),
    function() {
        echo 'form 2 complete, yay!';
    }
);
$formset->submitRequest(new Request());

FormSet.php

<?php
use \Symfony\Component\Form\FormInterface;
use \Symfony\Component\HttpFoundation\Request;


class FormSet
{
    /**
     * @var SplObjectStorage|FormInterface[]
     */
    protected $forms;

    public function __construct()
    {
        $this->forms = new SplObjectStorage();
    }

    public function addForm(FormInterface $form, callable $onComplete)
    {
        $this->forms[$form] = ['form' => $form, 'onComplete' => $onComplete];
    }

    public function submitRequest(Request $request)
    {
        $form = $this->getActiveForm($request);
        if (!$form || !$form->isValid()) {
            return;
        }

        // Invoke completed callback
        $this->forms[$form]['onComplete']($form);
    }

    protected function getActiveForm(Request $request)
    {
        foreach ($this->forms as $form) {
            $form->handleRequest($request);
            if ($form->isSubmitted()) {
                return $form;
            }
        }
    }
}