I think that you are mis-using the term "binding" which is a ZDO fuctionality specified by ZigBee which relates to indirect messge routing. I think that you are just trying to get network hops, which is really hard with TI's great radios and their incredible Rx selectivity ;)
So, you can add attenuators to the antenna mount, you can cable the boards together instead of using RF over the air, or you can just move the ZC and the two ZR's really far away from each other.
Also, you can connect Z-Tool to the ZC and put it in a disallow join mode, which will force ZR2 to join ZR1, but if your ZR2 is within radio range of the ZC, message routing will still not be what you want.
Thank´s for the clarification Dirty Harry.
How to connect Z-Tool to the ZC?
How to put ZC in disallow join mode?
I have to change any parameter in f8wConfig.cfg
best regards
Carlos.