There is no need for the hubs to be registered to the same account. You have two popular options for "linking" hubs. Both allow you to share devices over the LAN (or Internet) from one hub to the other, though to be clear, they do not form one giant Z-Wave or Zigbee network (if those are the kinds of devices you use, as most people do) but rather all form their own--this "link" happens over the LAN (or cloud if you want) and takes some work on all hubs. They also do not centralize hub administration--each hub retains its own admin UI you must use to configure that hub (and its apps, devices, etc.).
Assuming you are talking about devices: the native/built-in option is a pair of apps called Hub Link and Link to Hub. The popular community option is called HubConnect. The latter is far more powerful (may more types of devices supported, and if needed you can use custom drivers for specific attributes and commands), but it takes more work to set up--you have to install an app or two and at least one driver (plus more for any device types you share to that hub--and there are a lot of options there) on all hubs. Setting up also requires sharing (usually copying and pasting due to the complexity) a key between the two hubs. Hub Link/Link to Hub has everything built-in but doesn't support quite as much (and has at least one longstanding bug with RGBW bulbs not being shared correctly)--for example, sharing can only go one direction. HubConnect can share from either "server" or "remote" hubs. (Since you mentioned different accounts, I'll mention that I wouldn't do either of these if you don't fully trust whoever uses the other hub, especially with HubConnect where they can, for example, select new devices [of their own, not yours] to share to your hub from their hub without any action on your part; the limitations of Link to Hub might actually be better if this is the case.)
There are lots of other threads with people commenting on the differences and the pros/cons, and there's also nothing stopping you from writing custom code to share specific things between hubs yourself if you're up to that task (or using Rule Machine; when I didn't want to set up HubConnect--which I personally use on most of my hubs--on my development hub, I just wrote a rule to listen for an HTTP endpoint from another hub, which sent something there on a specific trigger).
If you're still not sure what's best, you might want to share you goals are (I ran with the idea that you were looking to share devices; if that's not it, feel free to specify). Then someone might have a better idea of what to suggest if the above isn't quite it. Otherwise, hope this helps!
