Wireless communication between multiple microcontrollers?
For communicating within a couple of meters range, these nRF24L01+ modules, at around $1.42 apiece on eBay,  would suit the requirements:



The module has a PCB trace antenna, which does a pretty reasonable job. 

The range seems to be stable at around 6-7 meters during the day, and I have occasionally seen as much as 20-25 meters late at night when most other electronic devices in the vicinity are off.

Sparkfun sells a similar module but with an SMA connector for an external antenna, which claims as much as 100 meters range at 250 kbps data rate. Of course, at nearly $20 plus the cost of an antenna, that may be overkill for the required 2 meter range.

Relevant to the specific application: 

The key improvement of the nRF24L01+ over the now obsolete nRF24L01 is the implementation of an auto-retransmit feature, which addresses the collision detection complexity concern raised in comments on the question.