How to select a specific Coordinator to join out of Two running at same time?

I am new to Zigbee and Xbee, so maybe this is a stupid question to ask. But I am missing the conceptual parts.

I am currently running XBee3 Mesh kit with two devices set as Coordinators.
I am setting the ID parameter as “0” so that they select randomly a PAN ID for themselves.

Now I have a router device, that will join to the coordinator.

How can I select which PAN ID the router should join?

I thought of doing the Network Discovery for the router, but then soon realized that it works only if you are on a network and then it lists all the devices in the network.