I have not found a way to show exactly the Input Dual Output Return fields as displayed in the action diagram variable palette (the only window that can be trusted and the generated source of course) via model api.
Obviously AutoTestBox - Stella Tools (CA Plex Developer Tools) relies on the interrogation of a function and creating its interface as meta data to a database.
I have managed this with a mixture of Model API and Meta but still know of a few ways that my code does not handle and neither does the (F11) Object Details - Parameters Tab in the IDE. And what to do with structure fields!
Have a look at StellaTools.AutoTestBox.UI.Wizard.Wizard.1_MaintainStub.CreateTriples in StellaTools model....and it is not for the beginner...
WHY DO YOU ASK as I might be able to think of a few methods to achieve what you want?