Two of my players' characters have been replaced by doppelgangers who want to steal money from adventurers (while letting them get into most of the danger), with the eventual object of escaping with the funds to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:
Pre-agreed code words or phrases which trigger particular events (these could be difficult to remember in practice).
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?