Have you heard anything about morse method?
Communication using morse method helps you to send your data to the receiver with a sequence of on or off messages with a light source.
This application do this job for you but instead of using a light source as your communication tool, you use the accelerometer of your device as the sending tool and the vibrator of the target device as the receiving tool of this model. The communication between devices will be done by their bluetooth devices.
Consider that you have installed this app on yours and your friend's devices. When you accelerate your phone, the shaking message will be sent to your friend's device and it will be vibrated and vice versa.

Application guide is included in the application package.