Hi,
Parallelism can be achieved as long as you run your activities/workitems asynchronously. Within the discussions if you search you'll find plenty of discussions on related topics.
i.e.
Re: saving state after workitemhttps://community.jboss.org/message/605996
https://community.jboss.org/message/649767
The documentation of jbpm is really great,
http://www.jboss.org/jbpm/documentation
The engine itself is single threaded (the same stands with other popular engines) as it should. . Howerver i think that is not what you're really after, what you need is your activities to be executed in parallel so async workitems will do nicely your job.
http://docs.jboss.org/jbpm/v5.3/userguide/ch.core-basics.html#d0e2327 (also take a look in the asynchronous handlers section etc)
http://docs.jboss.org/jbpm/v5.3/userguide/ch.domain-specific-processes.html#d0e7113