Once two client's sessions are linked you can then retrieve the sessions using getLinkedSession.
private void sendByeToServer(SipServletRequest clientBye)
throws ServletException, IOException {
B2buaHelper b2buaHelper = clientBye.getB2buaHelper();
SipSession serverSession = b2buaHelper.getLinkedSession(clientBye.getSession());
SipServletRequest serverBye = serverSession.createRequest("BYE");
logger.info("Sending BYE request.\n" + serverBye);
serverBye.send();
}