If you don't create a separate network, my only concern would be that if something happens to your "main" hub and all your HSM sensors are linked to your "HSM hub" via HubConnect, then that connection is unlikely to work, either. So you'll have an HSM hub that is running fine, but possibly nothing coming to it to do anything when you want. (Obviously this is more of a worst-case scenario, so in other cases just having "problems" with the main hub won't necessarily be bad for the other, but then the automations would probably run just as well on that hub, too.)
That being said, I'm not sure I have room to talk: I have a "second" hub dedicated to lighting, but to avoid having even more Zigbee networks, it's actually also my (only non-test and excluding Hue) Zigbee hub. Things that aren't lighting-related (like when I had a Zen Thermostat) go off to the main hub via HubConnect. Some Z-Wave devices on the main hub come to this one via the same. But all the important sensors for lighting are Zigbee (Z-Wave is either where I don't care about speed or don't care much if it temporarily stops workings at all, like a sensor for keeping lights on rather than turning them on). Many of the Zigbee sensors here still go to the other hub for other purposes, including HSM. My goal was to keep lighting automations as fast as possible by letting one hub do almost only that. Still not perfect, but I think at least disturbing the "real" devices among the two means that something will work even if the other doesn't work fully.
My preference, of course, is to keep both working and figure out the cause if not. But separating my lighting to keep it as fast and reliable as possible is easy enough.