yes this is possible, just do it... nothing special to it
By "sibling" do you mean "fire and forget" - that is, the first process starts the second and then doesn't wait for it or communicate with it again?
If so, then:
- it's not modeled at all, but,
- it's really easy to write an ActionHandler that will do this. If you want, the name of the subprocess can be in the parent process (though I don't think the graphical designer will do this yet - I think you'll have to hand-edit the XML).
Check out the examples, tutorials, wiki, etc. Above all, read the User Guide... then read it again.