UML22 — show object flow or interactions

There needs to be a way to show object flow or interactions between multiply concurrent threads or processes in Activity Diagrams. Example: In TCP sockets, the interaction between a client and server should be able to be shown with two separate start points, one for the client and one for the server. The connection sequence and packet flow should be able to be shown. With a single start point, the diagrams imply that one action starts both processes. I would like to illustrate multiple concurrent threads or processes and their interactions in an Activity Diagram and be able to distinguish between the flowing threads. I would also like to show access to objects shared by the threads or processes.

This issue has already been discussed in previous F/RTFs and considered out of scope:
FTF: Activity diagrams only show task dependency, which can be achieved by multiple implemented processes. An activity can have more than one initial node. These are all started when the activity is. The initial nodes can be used in separate partitions to indicate which actions are taken on the client and server. If the two processes are completely independent, then this is a request is for a hybrid diagram, especially when trying to show shared objects. This is too much for an FTF to address.
RTF: Hybrid diagrams are too complicated a topic for an RTF to address. There are many combinations and not enough experience to choose among them.
Revised Text:
None
Disposition: Closed Out of Scope