Adding the two conductors between the two XP8s will only transfer trigger codes from an RM-433 or in-wall, not a ZM-24, which uses the COM port. You need a ZM-24 per XP8. Also, the ZM-24s will "cross communicate" to each other per these statements in the ZM-24 installation tech bulletin (please note I haven't done this myself yet). Also, note this tech bulletin was updated Feb. 2010 so this info may be new to most of us.

1) An RTI zigbee network has a main ZM-24 that selects the network channel. Any additional ZM-24 units in the system (both wireless “repeaters” and hard-wired ZM-24’s connected to other processors) will attempt to search for the network channel information provided by the main ZM-24 transceiver. Therefore, if multiple zigbee enabled RTI control processors (XP-8, MRP-64, ZRP-6) will be utilized within the same Integration Designer system file, it is critical that the ZM-24 transceiver modules are located in an orientation which allows zigbee network information to be transferred.

2) When multiple Zigbee enabled RTI control processors will be utilized within the same Integration Designer system file, each processor must have a ZM-24 wired to the RTI COM port. In this configuration, it is extremely important that all ZM-24’s are within communication range of each other. Please see the section on setting up a Zigbee network using multiple processors (page 4).

3) If there will be two or more Zigbee enabled RTI processors that are located remotely from each other, and don’t require any cross-communication between systems, it may be beneficial to program the processors in separate system files. This eliminates the need for the two systems to be connected to the same zigbee network.

To me this sounds like you can have two XP8s, each will its own ZM-24, and I can be in one room closer to XP8(a) and ZM-24(a). Remote sends command to ZM-24(a), ZM-24(a) repeats command to ZM-24(b). Then ZM-24(b), which is hard wired to the XP8(b), executes macro.

Dino