I don't know about the M2000 in particular, but in general; transmit mode = range only, receive mode = bearing only and T/R = both. As explained in the Mudspike article, TACAN is basically a military VOR+DME. A DME requires bidirectional communication between the interrogator (aircraft) and transponder (ground station), while VOR receivers just receive the FM+AM VOR signals and compares the phase between the demodulated carrier and sideband in order to get the bearing to the station.
For the simple A/A mode (with just ranging) one aircraft acts a DME interrogator, while the other one acts as the transponder.