Two of my players' characters have been replaced by doppelgangers who want to steal money from adventurers (while letting them get into most of the danger), with the eventual object of escaping with the funds to set up a fraudulent merchant business. The players are aware that they are now playing doppelgangers, and happy to go along with it.
Doppelgangers usually try to isolate party members before killing them, and, in this case, taking their gold. The problem is that all action is usually narrated by the DM. A possible solution is that I narrate some pre-agreed encounter, which is almost guaranteed to kill the target, but which is not, in fact, happening, and which can then be reported back to the party by the doppelganger as the reason for the unlucky character's demise.
In another ploy, the doppelgangers may wish to strip treasure from undiscovered rooms before the rest of the party get there. Once again, I have to know not to narrate the discovered treasure, but somehow tell the doppelgangers what they have actually found.
The players may also come up with other schemes that require coordination between us, or between themselves, without the other players knowing. Passing the notes telling them of the change has already aroused quite considerable suspicion, so this is not a workable solution.
Ideas we have already thought of include:
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Pre-agreed code words or phrases which trigger particular events (these could be difficult to remember in practice).
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Sending each other messages on our phones during breaks (does not allow communication during the session).
How else can the two doppelganger players and I coordinate the deception?
Best Answer
Take inspiration from real-world spies.
Your doppelgangers are essentially operating undercover and attempting to communicate without being intercepted or spotted by other members of the group they've infiltrated. Why re-invent the wheel when Cold War era spies conducted similar activities for decades?
Real-world spies could often not communicate with each other freely; they had to arrange meetings, or dead drops of information and items, and might go for long periods of time without actually interacting with each other. They might not have reliable methods of communication. Players acting as spies should expect that they may have to do the same.
In private messages, task your doppelgangers with the responsibility of finding their own methods and opportunities to communicate without being overheard or understood. This creates an interesting challenge for the players and rewards them for clever gameplay. Their real-world equivalents often had to think on their feet.
The players can leave messages for each other hidden in a pre-arranged place in the bathroom.
During the game, the players can signal the DM or each other by certain things which normally appear innocent:
Finally, and perhaps the best advice, I recommend playing a "hidden identity" board game such as Secret Hitler, which will give players practice in conveying information without revealing which of them are secretly bad guys.