This turns out the be a pretty hard problem. My first guess would be to use some kind of IR beacon on one, and sensor on the other, and put a tube in front of the sensor so it only senses a particular direction, and then put that whole sensor assembly on a servo that you can swivel around. That will give you direction. Add an ultrasonic sensor, perhaps, to get approximate distance? Assuming the remote Arduino is big enough to give a ultrasonic response.