As I would see the configuration:
- knowledge base is loaded on both servers from a shared repository, Guvnor for instance using knowledge agents
- sessions are maintained per server with persistence, can be reused across requests for the same user (just an example of session reuse)
- task service is available for every server as independed service
- load balancer is distributing load to app servers based on session affinity so same user will be directed to the server where his/her session is still active
Just to summarize, what was discribed on these few posts I think should allow you to cluster jBPM where only db and knowledge base repository are shared between application servers.
Good luck