You could do this using the Odoo bus adding the behavior of processing an specific message to set it in the odoo session and cut the message propagation. You need to have a mail.channel with the other user. But think that the session for the user is not a matter of having the uid of the user, if you have the session id of the user session you could just retrieve it from the session store but if not, and I think that is the case, you need to communicate with the user session and the only way I could imagine is using the bus.bus messages because also if you have a way to login using xmlrpc into the user account you will end up with another session, not the one from the already authenticated user sessions, yes there could be multiple users sessions. With the bus.bus you could reach all of them or just one depending on the info that you pass it