How can multiple z-wave hubs work together?

I have an ISY994i which has a Z-wave dongle. At the moment I only have one Z-wave device, which is a Schlage Connect lock which is part of the ISY’s Z-wave network. I don’t know how I did it, but I think I made my ST hub a “secondary” controller.

Am I correct in my understanding that a z-wave device can only belong to one network? I can see my Schlage lock in both the ISY hub as well as ST, though it seems like I have 2 Schlage locks in the ISY. Would that be because the ISY is the primary and the ST is the secondary and I was able to add the Schlage into the ST which was in turn found by the ISY?