One app! keep to one app for sure and control access with Role Permissions.
While building your first app may have been painful, they will get much much easier the more you build. QuickBase is like 1,000 easy things to know, but you need to make 1,000 mistakes once. Then you will be flying.
Believe me it gets old in a big hurry to make the same changes to the app twice. it's sort of fun and satisfying once (well for me anyways), but definitely not fun and not satisfying doing it twice. You may think that your app is finally perfected, but only if the world stops for you and nothing changes. But the world does not stop.
You need to be agile 'cause as soon as yer app is done, word will comes down that the needs of the business have changed, and you need to adapt the app to meet the new needs. Quick Base is a journey and not a destination. Any user with half a brain will "get it" and tell you "this is good, so thank you, but if we could do this .. it would be better". They will keep pushing the envelope and scope of your app.
