When you look here and there on the web you’ll read that multiple coordinators in a Zigbee network is not possible, but the Zigbee Spericiation (2.3.2.3.6 MAC Capability Flags Fields) indicates there is a bit "Alternate PAN Coordinator “this node is capable of becoming a PAN coordinator.”
So in full, there is only one PAN Coordinator at at time but it looks like there can be devices that could be come the coordinator.
Possibly this is a MAC sub-layer requirement and this was not specified for the Zigbee network.
I could not yet find an official document explaining how that works, but there was mention of it in microchip’s code.
So I assumed it’s possible because of that mention in the specification.
(Un)fortunately there is a paper about it though.
This suggests that there is no official method, but that it’s not impossible to do.
I had a quick look and it proposes to have a second device which acts as “backup router” that “pings” the coordinator and takes over after several unsuccessful pings.
The coordinator also sends network settings to the backup on a regular basis.
The switch implies that the backup “restarts” to the coordinator software and reuses the coordinator’s IEEE address.
So based on that, the Zigbee 2008 specification did not define how an “online” backup would work. And I do not think that has changed in the latest documents.
So there is no official method for a backup coordinator, but there is a proposal for what could be called a hack.