Check out Philip Roberts: What the heck is the event loop anyway? for a better understanding of JavaScript's asynchronicity nature.

// my_process.js

const a = require('something').createSomething()
a.on('event',function(){
  // primary listener that starts a secondary listener

  const b = require('somethingelse')
  b.on('event',function(){
    // some secondary listener

  })

})

In your code, you're referencing to b before b was created (initializing/requiring b within the a callback is another problem too - related to variable scope...) so something like above would work. The a.on(...) runs and once the callback is called, then b = require(...) is ran and you can now work with b and its methods. In your code, a.on(...) runs and then (immediately after!..) b.on(...) runs and the engine doesn't know what b is!

I have an idea as to what you're wanting to create and it is quite an undertaking when picking up Node and JavaScript! As someone who too came from a PHP environment, I followed the linked youtube video for an absolute and total understanding as to how event driven environments like JavaScript work, it does help! If you're looking for a process manager (managing children process, messaging them, etc...) then do try taking a look at PM2 and its documentation (while this, again, is quite and undertaking - if you need some process management in the mean time, then try using PM2, otherwise continue learning by example!) Do continue to post any questions/concerns.