title: How to set a downstream task after finish of 2 Tasks
body: <p>I am running tasks(T1 and T2) in parallel. How do i set a downstream task T3 which depends on both T1 and T2 being finished successfully</p>
  
  <p><code>[T1,T2].set_downstream(T3)</code></p>
  
  <p>But, getting following error <strong>AttributeError: 'list' object has no attribute 'set_downstream'</strong></p>