Thank you for your detailed answers and example.
I understand what needs to be done.
It would have been nice with a possibility to define this in the structural part of a model (having extended xtUML semantics for this subdelegation), rather than having to specify extra behavior in the model.
The extra behavior will affect the scheduling of the state machines unnecessarily.
Typically there is support for this in most domains / operating systems where a model will be deployed. Anyway, it is not that difficult to implement generic model integration code that implements such an extended xtUML semantics.