Hi,
in addition to example given by Huisheng Xu you can find some interesting information about rules and rules decision activity in dev guide:
http://docs.jboss.com/jbpm/v4/devguide/html_single/#rulesdecision
I am nit sure I got your point but in my opinion you could do it with fork and join. Meaning that to execute a subprocess and main process in pararell you use fork where one branch is the subprocess and the other is main path. After you join them and do it quite the same for second subprocess. That way execution will wait in join until both branches are finished.
HTH
Maciej