Multiple actor systems for an application
<p><a href="http://manuel.bernhardt.io/2016/08/23/akka-anti-patterns-too-many-actor-systems/" rel="noreferrer">This</a> article talks about how we should not create 'too' many actor systems. But the <a href="http://doc.akka.io/docs/akka/2.4.1/general/actor-systems.html" rel="noreferrer">docs</a> say:</p>

<blockquote>
  <p>An ActorSystem is a heavyweight structure that will allocate 1…N
  Threads, so create one per logical application.</p>
</blockquote>

<p>I am unable to understand what is the real issue here with using multiple actor systems in an application. Also, is it possible for actors from different actor system to message each other?</p>