MultiChain does not (yet) provide a mechanism for off-chain messaging between nodes. So you either need to deliver the partially signed transaction outside of MultiChain, e.g. using web services, or you can just post it into a MultiChain stream which the other party is subscribed to. In that case of course the partially signed transaction will be visible to (and stored by) all participants' nodes, and you'd have to decide if that's OK in your case.
