Besides the above, it should be noted that if you're coming from SmartThings, the Hubitat vs. SmartThings app serve quite different purposes. Notably, the Hubitat mobile app is entirely optional: it's one option for presence and notifications, but there are lots of community and a couple other natively supported options. It's also an easy way to access Dashboards, but those are ultimately just webpages that you can access from any browser with the right link.
The administration/setup functions from the ST app is all done via Hubitat's admin UI, which is a local webpage served from the hub itself. This is how you can add/remove devices ("Things," if you will), add/remove and modify apps ("SmartApps," if you will), as well as add custom app or device code and view live or past logs (IDE functions in SmartThings, except the last thing that they don't have at all). The ST app also does device control, and the admin UI can do this but isn't really meant for doing it all the time--it's not pretty, just an exposition of all device commands and preferences and some other relevant data. The intended day-to-day solution for this is Hubitat Dashboard, or whatever other option you might want to set up like SharpTools or just automating as much as you can so this isn't much of a need at all.
So, you can definitely view multiple hubs' admin UIs--you don't even need the app for that. If you aren't at that particular location/network at that moment, however, you would need a VPN or other remote-access solution to actually view it. But again, that's just for administration. A Dashboard would let you view and control devices, change mode or Hubitat Safety Monitor arm status, and (unless disabled) view device history--just about anything you want except hub administration.
Back to the app, you can't associate it with multiple hubs at the same time, but you can switch between hubs. However, that would affect notifications and presence if you use those features, so if you do want to use the app, picking one hub to associate with the app and finding another solution for these features on another hub would probably be a good idea. The only good I can see the app doing you when away is easy Dashboard access to view/control devices, again something you can do without the app at all, so you could just use the cloud (and/or LAN) Dashboard links from the other hub--or both if you prefer--to get that.