I have been searching and Googling for this the past few days and after a very nice support chat with Engine Yard I found out:
It is possible to run multiple applications in the same environment on Engine Yard AppCloud. So, you can add an application to an existing environment.
I knew this was possible before, but I couldn't really find it in their documentation, hence this blog post.
You can just create your first application and create a single-instance, cluster or custom environment for it and than add other applications to it by pressing this button:
If you want DelayedJob, Resque or Sphinx for one or more of the applications in that environment you will need to do some manual Chef configuration and I will write about that soon.