Hi Peter,
in fact that would not be a problem if jBPM has two types of nodes that converge executions (exclusive and parallel). To be honest I was thinking as well to introduce new type of join node but it would be only to have capability of informing the engine if it is exclusive or parallel - exactly the same what this new attribute does.
I agree that deciding about what kind of join it is is responsibility of the engine. I will take a look at possibilities of implementing what you described and post back the result.
Thanks a lot for very good ideas and engagement in this.