I just updated the documentation with a section about multi-threading. Internally, the engine is using only one thread to execute a process instance but you can achieve logical parallelism by using asynchronous tasks:
http://docs.jboss.org/jbpm/v5.1/userguide/ch05.html#d0e1808
So in this case, make sure that your self-defined service is invoked asynchronously and you should see them being executed in parallel.
Kris