You can search the forums for a few ideas, but a true "hot spare" isn't really possible, especially if both are C5s with internal radios. Zigbee devices remember the ID of the coordinator they are paired to, but the hub has all the necessary information in the database, so a re-pair of the device with a hub database restore will be sufficient to put it "back in place," but will still be necessary (there's little else you can do since the coordinator ID will change with the different hub, and the devices need to "know" that, but honestly that's pretty painless in the grand scheme of things). Z-Wave stores some information on the radio side, and the current hub backup tools cover only the hub database, not this. A re-pair will create a new node (far less painless).
An external stick would help in both cases: if it doesn't die, you could theoretically just move it with a backup to the new hub. To cover if that dies, third-party tools can do a backup of an external Z-Wave stick that you can restore to a new one, but I'm not sure how many people have tried or had luck with that.
