title: Use data from different pipeline
body: <p>I have two pipelines, "gameEngineEvents" and "userEvents" that consume from equivalent pubsub topics. A userEvent might have one or many gameEngineEvents.</p>
  
  <p>When a gameEngineEvent happens I want to check if there is a userEvent that has a reference to that gameEngineEvent, run some logic and then publish a new message to a third pubsub topic.</p>
  
  <p>So, is it possible to do something like this only in dataflow?</p>