I don't think it's (easily) possible, because Jingle is a client-to-client protocol.
Maybe the server could initiate the Jingle session in behalf of one client and also accept it in behalf of the other client, but then there's still the negotiation process, which needs to be done.
E.g. the server cannot know, if a client is behind a firewall and cannot connect to a given stream host. Nor can it know the local streamhost a client might advertise.