You can use only one schema for both task and process persistence units.
Please remove the database properties from the process persistence unit as the datasource is going to be there. I put those there by mistake. Though it wont make a difference.
Change the value of transaction lookup class property hibernate.transaction.manager_lookup_class from websphere to jboss.
You will need to create a datasource in jboss same as the name mentioned in the persistence file.
These are the changes you need to do.
But the error you are getting is because the name of the persistence unit i.e. org.jbpm.persistence.jpa is being used by another project which is also deployed in the same container. So Make these changes and then deploy the archive in another jboss server which has no other projects just to be safe.
Regards.