Adding another hub to an existing ZWave results in the new hub being what ZWave calls a Secondary Controller.
The list/map of the devices existing on the primary gets copied to the secondary, and that's it.. never again. The next ZWave device you add will probably only appear on the primary hub. The secondary will know nothing about it. You could pair it via the secondary and in most cases the secondary will inform the primary in the sense of acquiring the next ZWave address from the Primary.
After the original pairing of the two hubs, you would be able to use either controller to turn devices off and on, or brightness, etc. What would probably NOT occur is that a door sensor wouldn't tell the secondary. The Door sensor would have been paired with the primary, would never learn of the secondary and thus would never have a reason to send a packet to some "invisible (to it)" device.
Polling by the secondary would eventually have the states sync, but by then, perhaps something else would be out of sync.
If there's a mechanism for the Secondary to send it's list/map to another device (Alexa/Google Home, etc.) then that would occur, resulting in doubling of devices on Alexa.
On the day I purchased my first Hubitat hub, 18 months ago, I had 4 Hubs in exactly this situation.
I had a SmartThings Hub joined to a Staples Connect hub and a Aeon ZStick (running Open Remote) all joined to one ZWave network. By carefully poking the join buttons, I could get most of the devices to all of the hubs. I also has a Wink1 hub joined, but it was a serious disappointment and I never bothered using it for anything. I pulled the Hubitat out of it's box and Joined it as a Secondary the same as the other 4.. for a total of 1 primary and 4 secondaries. Keeping them all up to date was tedious but for the most part, I wasn't adding new devices.
I left it that way for maybe a month before concluding that Hubitat would do all I needed. I could see the future where I had only ONE hub. I did the standard migration of Excluding devices from the old network and Including them to Hubitat. I did it in a day, but no one recommends that. I had pretty big advantage because I already had all the Rules needed on Hubitat. I just had to pair a device and then get it back into it's rule(s).
I took that path so you wouldn't have to
Fast forward 18 months and I'm back to having 5 hubs. Three Hubitat, a SmartThings and Homebridge, all tied together via HubConnect.