XBee S1 addressing question

I am building a radio control system for an outdoor model railway, a project purely for my own interest. It has two hand-held controllers, each with a PIC and an XBee, and several locomotives, also each with a PIC/XBee pair. My original plan was to use broadcast mode, with addressing handled by the PIC firmware, and relying on CCA to prevent simultaneous RF transmissions. Messages are 7 bytes long, transmitted every 50mS. The 50mS is actually a fixed 43 mS plus a randomly-generated 0-15mS as an extra precaution against the two controllers being in sync. With one controller working everything is fine, but if the second controller is working I get serial comms errors in the data from the XBee to the PIC in the loco. I get enough uncorrupted frames to work OK, but I’m not happy with the setup.
I could give each loco a fixed channel number, and change the controller XBee’s address to suit the loco I want to control, via AT commands. The downside would be a limitation of 12 channels, 0xC to 0x17, and I would have liked more.
Can anyone tell me why CCA is not suppressing the simultaneous RF transmissions?
Can anyone suggest a workable addressing scheme that will allow more than 12 receivers?
I’m very comfortable writing the PIC firmware, but I’m out of my depth when it comes to networking.
Thank you.

First off, clear channel assessment is only going to check the channel when it begins to transmit. If both are truly at the same time, there will be data collisions as the CCA will not see anything transmitting when that function is checked.

You can always use the 16 bit addressing function instead of broadcasting out everything. Then only radios that are on the same channel, PAN ID and have the correct value for the DL in their MY will receiving and out put the data.