lookup tables across apps

Yes, you can.  It's called a cross app lookup.  

Here are the main caveats.

1. You can only create the relationship from the Child App.
2. You can only see the relationship from the child side, so sometimes that can be confusing.
3. The big factor is that in doing so, the two apps will be treated as one larger app by the servers and be server by one "server process".  Quickbase is really Quick,  but it only does one thing at a time.  So if you have a lot of data, like tables with say 100,000 's of records or say 100+ users then instead of getting two server to run those two app, they get single threaded in one server process.  So if someone runs a report that takes 3 seconds to load, everyone on both apps waits those 3 seconds for a simple "Add Record" to save.

Best Practice would typically to make a Connected Sync table to mirror the data from the Parent into the app, and then you do not create app dependencies and you still keep the apps separate for performance.  It's a real pain to untangle apps once they become dependent if you start to run into performance issues down the road in a few years. 

An advantage of a connected table could be this situation: Let's say you want to have a single source of truth for your Customer Master. Yes you can do a cross app look up where you need to pull down a customer into some kind of record but that means that app that app won't have its own table of customers in it. But if you set up a connected sync table it becomes more natural for the users to be able to find customers in the second app, and the customer record form could have some convenient embedded reports.