If your goal is just to "control" devices, you don't need the app at all: just make a Dashboard and use the direct web link, local or cloud, without the app, at least for one of the hubs. (You'll need a dashboard to control things using the app, anyway, unless you're just using it to navigate to the admin interface, in which case you definitely don't need the app but I'd also recommend something designed for this purpose. like a dashboard.)
But to directly answer your question, you can only have one account associated with the app at a time. So, either you'll have to sign out and sign in every time you want to switch--probably not what you want--or find a way to use the second hub without the app. If you don't need any Hubitat app features the second hub (besides Dashboard access), like presence or notifications, then you should be fine without it. If you do need those other features on both hubs, they'll only work for the currently-signed-in hub, so you'll need to use a different solution for one of the hubs (like Pushover for notifications or OwnTracks, Life360, Alexa, HomeBridge, or something else for presence).