@ Fozzy. Depends on your string material and how tight it is. What's to stop a third actor tying on to the string at any point between cups?
Also I suspect the same laser technology used to evesdrop via window glass could work with the vibrations in the string, best thing is don't talk to or communicate with anyone.
Is the Oz government intent on making all Aussies into criminals? The laws there seem to be getting tougher and tougher