This largely depends on the availability of APIs. I believe both Amazon Echo and Cortana now have APIs available. Whereas, Siri and Google Now do not. (The Google Now “cards” API is not a real conversational API.) Conversational AIs with APIs can be relatively easily interconnected with minimal coding. Otherwise, Siri and Google Now would have to be mechanically interfaced, in other words connected mic-to-speaker and vice versa, either hardwired or acoustically. However, I can tell you that such conversations would not be very long, and would have to be initiated by a human intercessor – because, neither Siri nor Google Now are pro-active.
See also my quick and dirty webpage: