Actually, I think you should see them as two networks with two different Kademlia distributes hash tables. I think that as long as they use the same login protocol (I think IRC channels are often used for this?), when people log off and try to log on again, there’ll be a random chance proportial to network size of your vault trying to connect with one or the other network.
If you happen to try to join the “wrong” network (which you weren’t previously part of), the consensus group of that network that handles your joining will reject you due to unknown data being in your vault (or your passport may not exist there, but let’s ignore that for now). If you happen to have no data that is unique to one of the two networks, you can probably join either, until one of them puts unique data in your vault. Then afterwards you can only join that network.
So actually I think the networks wouldn’t try to merge, there would just be chaos for (re)joining vaults that would randomly (proportional to size) try to join one of the two networks, until one of the networks decide to change their joining protocol somehow (or their members decide to abandon their network and join the other one).
Anyway, don’t take my word on this. I may very well be wrong, but I think this is how it would work.