I just had the XL824 installed yesterday with a new HVAC unit and I'm not sure if its going to work, at least with the current software it has.
The XL824 includes a Nexia bridge and you have to enroll it with Nexia to even enable the Nexia/Zwave menus. At that point I can get options to include/exclude other Z-wave devices, but don't see any options to include the thermostat as a device on another controller. The manual talks about being able to put the bridge into a "repeater" mode if you have another Nexia Bridge on your Nexia account, but that would involve an investment in the bridge and the monthly investment in the account.
Its looks like this is the initial rev of the thermostat software, so its probably something that could be added considering most of their other thermostats can be added that way.
If there is anything anyone wants me to try playing with, let me know.