Z wave devices can only belong to one network at a time, and each network can only have one primary controller.
It is technically possible to add another controller as a “secondary” but to be honest SmartThings just doesn’t do this very well as either primary or secondary. So support officially recommends against it. And they will not help you if you mess up your network by trying to make it work.
I also find that most people don’t understand what being a “secondary Z wave controller“ means. It does not mean that suddenly both controllers have access to any information about all of the zwave devices on your account.
Instead, it means that at the time you joined the secondary controller to the network, it will get a copy of the network device table for all the zwave devices that are joined at that time, and it will be allowed to issue commands to most of those devices ( but maybe not locks and other devices using security protocols). That’s all it means. It doesn’t mean that when a switch comes on both controllers will be told that the state has changed. It doesn’t mean that the secondary will be told about any new devices that are added later. There are some systems, such as vera, which have their own software to keep all of the primary and secondary hub information in sync after the initial joining, but that’s not required by the Zwave standard, and smartthings doesn’t do that. Nor do most of the security systems that use zwave.
So your best bet is to search the forum for other people who might’ve tried it with the Honeywell system and see what their results were. It might work, it might sort of work, it might not work at all – – it just depends on some specific details of how that manufacturer has designed their platform.