Given the following:

        /// <summary>
        /// Received client token.
        /// </summary>
        public static void HandleCityToken(NetworkClient Client, ProcessedPacket P)
        {
            try
            {
                bool ClientAuthenticated = false;
 
                using (DataAccess db = DataAccess.Get())
                {
                    string Token = P.ReadString();
 
                    foreach (ClientToken Tok in NetworkFacade.TransferringClients)
                    {
                        //Token matched, so client must have logged in through login server first.
                        if (Tok.Token.Equals(Token, StringComparison.InvariantCultureIgnoreCase))
                        {
                            ClientAuthenticated = true;
 
                            Character Char = db.Characters.GetForCharacterGUID(new Guid(Tok.CharacterGUID));
                            if (Char != null)
                            {
                                lock(NetworkFacade.CurrentSession)
                                    NetworkFacade.CurrentSession.AddPlayer(Client, Char);
 
                                PacketStream SuccessPacket = new PacketStream((byte)PacketType.CITY_TOKEN, 0);
                                SuccessPacket.WriteByte((byte)CityTransferStatus.Success);
                                Client.SendEncrypted((byte)PacketType.CITY_TOKEN, SuccessPacket.ToArray());
 
                                break;
                            }
                            else
                            {
                                ClientAuthenticated = false;
                                break;
                            }
                        }
                    }
 
                    if (!ClientAuthenticated)
                    {
                        PacketStream ErrorPacket = new PacketStream((byte)PacketType.CITY_TOKEN, 0);
                        ErrorPacket.WriteByte((byte)CityTransferStatus.GeneralError);
                        Client.SendEncrypted((byte)PacketType.CITY_TOKEN, ErrorPacket.ToArray());
                    }
                }
            }
            catch (Exception E)
            {
                Logger.LogDebug("Exception in HandleCityToken: " + E.ToString());
            }
        }
 
 
(...)
 
        private Dictionary<NetworkClient, Character> m_PlayingCharacters = new Dictionary<NetworkClient, Character>();
 
        /// <summary>
        /// Adds a player to the current session.
        /// </summary>
        /// <param name="Client">The player's client.</param>
        /// <param name="Char">The player's character.</param>
        public void AddPlayer(NetworkClient Client, Character Char)
        {
            lock (m_PlayingCharacters)
            {
                foreach (KeyValuePair<NetworkClient, Character> KVP in m_PlayingCharacters)
                {
                    ClientPacketSenders.SendPlayerJoinSession(KVP.Key, Char);
                    //Send a bunch of these in reverse (to the player joining the session).
                    ClientPacketSenders.SendPlayerJoinSession(Client, KVP.Value);
                }
 
                m_PlayingCharacters.Add(Client, Char);
            }
        }

why do BOTH players receive information about the other (or self?) when I log on using the same account, but NONE receive any information when I log in using two different accounts o_O

The answer should be blindingly obvious, I can't see it.