title: Two data streams in one flink job
body: <p>I have two infinite kafka data streams in one flink job, like this:</p>
  
  <pre><code>StreamExecutionEnvironment environment = StreamExecutionEnvironment.getExecutionEnvironment();
  DataStream kafka1 = ...
  DataStream kafka2 = ...
  
  kafka1.process();
  kafka2.process();
  
  environment.execute();
  </code></pre>
  
  <p>Does the process of the two streams are parallel or in sequence?</p>