If I can constrain two robots to a hypothetical box, what sensors or common formal methods exist that would enable the two robots to meet? I am specifically interested in the communication of relative position between robots.
I would recommend using the XBee protocol to develop a point to point communication unit. What you need is an xbee adapter - which contains both a transmitter and a receiver. You can transmit data serially between the two robots which allows you to send the position of each robot. The implementation details (error checking, parity codes, parsing) are abstracted away from you when using the Xbee and you can set up an xbee connection between two robots like a serial connection.
If you are focussing on getting two robots to meet, you can transmit their position with respect to a fixed coordinate frame in the box using two Xbees and then calculating the relative position of one robot from the other robot and control their heading to intersect each other's center of mass. This way you are guaranteed that the two robots will move towards each other. This is not an optimal way to get to robots to move towards each other in the presence of obstacles - in which case you should go for more sophisticated path planning algorithms like A*, D* Lite etc.
The range is governed by the type of antenna, the transmitting power and the frequency of transmission. Lower frequency com equipment tends to have larger penetration power and thus more range. However, legal restrictions on the frequencies civilians can use depends on the country you are in. (900 Mhz is legal in the US but illegal in India. 2.4GHz is legal, free to use ISM band worldwide). The more your transmitting power, the more the range you will get but this too is regualated by the RF restrictions in your country.
Found a great platform just by Googling "communication between robots" called Jasmine. By "great", I mean well documented. I am reading through their Communication documentation, now. It starts out (in the very first paragraph) by saying:
Communication works different in most robotic swarms, depending mostly on factors like the environment, the size of the robots, the budget of the project and on the limitations set by creators or other instances. The larger the robots are, the more possibilities are there for building up a properly working communication between the robots. Some ways of communication are bluetooth, wireless LAN, communication via the environment(stigmergy) or like in the case of Jasmine via infrared LEDs.