I believe what you do is you record an hour of normal conversation and then play back dozens of timeshifted copies simultaneously. Any system is going to have a very hard time filtering out noise with the same statistical properties as the signal - including you. You might have to stand quite close together...
More realistically, a bathroom shower; the water plus the echoes from the tile/glass in typical bathrooms are better white noise than any digital solution. Two pots boiling at different heats also works and you potentially get the benefit of nutritious meal. Many kitchens also have noisy extractor fans conveniently located right above the stove!
Working in film, you want sound to be realistic but viewers have to be able to link it to something on the screen. Those loud hums from store/restaurant refrigerators used to be the bane of my life because while they were often the authentic natural sounds of the environment they're rarely visible unless you are specifically shooting a scene in a meat locker or next to an airport.
If I was shooting a remake of The Conversation for the digital age and my secretive chatters weren't using high tech wizardry, I'd give them two am radios tuned to bible or foreign language channels. 3 overlapping sound sources recorded at a single point (without the direction resolution our ears provide) are possible to disentangle, but it's a ton of work. Speech is harder to filter than music.
If you're in real fear of aggressive surveillance but must communicate, pick the noisiest room (usually the kitchen), sit on the floor or under a table, and hold your conversation inside a large cardboard box with some thick blankets over the top. I would sometimes use this when I needed to record in a noisy environment where it wasn't practical to run wires.
Thanks for the movie. The kitchen suggestion reminds me of the joke:
- Did you know the soviets invented the Internet?
- What? Are you high?
- It's true: we used to sit in the kitchen in our pajamas, talking about everything but never getting anything done. The only difference nowadays is that the illumination is cold white instead of warm orange.
