[svjx]

Hmm... so just off the top of my head, I'm thinking one service would have to forward to the other. Only problem with that is that the sending server would now be the relay and not the original sender. Since MX records only account for using one service, there would need to be some sort of intermediary to do this. Possibly have mail delivered to a common server which then sends it to both Mango and Google (giving each the same odds to detect spam).

If you do end up trying it, feel free to shoot me an email either at [e-mail] or my personal, [e-mail]