title: Asynchronously run different animations in manim
body: <p>I'm trying run essentially two animations (ref. following code):</p>
  
  <pre><code>class RelTrain(Scene):
      def construct(self):
          train = Rectangle(height=1, width=4)
          train2 = Rectangle(height=2, width=2)
          train.move_to(np.array([-10,0,0]))
          train2.move_to(np.array([0,0,0]))
          self.add(train, train2)
          self.play(
              train.move_to, np.array([10,0,0]),
              train2.move_to, np.array([15,0,0]),
              run_time=18,
              rate_func=linear,
          )
          self.wait()
  </code></pre>
  
  <p>Essentially two rectangles are moving, but I do not want them to begin movement simultaneously. I want <code>train</code> to start moving, and after 2 seconds (<code>train</code> would still be moving at this point since <code>run_time=18</code>), I want <code>train2</code> to pop up on the screen and begin its motion. I'm not sure how this is done and would appreciate any help.</p>