Yes, certainly you can reference data stored in one FileMaker file from another. You can do this a couple of ways.
The most direct way is simply to add a table occurrence (TO) to your relationships graph that refers to the other file. Here's how:
1) Go to File > Manage > Database.
2) Choose the Relationships tab.
4) When the "Specify Table" dialog appears, choose "Add FileMaker Data Source" under "Data Source":
5) Follow the path to the FileMaker source. (Use the "Remote" button to point it to a file being hosted on Server.)
The other method would be to add the data source manually. Here's how:
1) Choose File > Manage > External Data Sources.
2) Click "New".
3) In the "Edit Data Source" dialog, give the data source a name (typically, the name of the file) and then click "Add File" (to point it to the file you want to add).
Then, add the TO to your Relationships Graph as you need to.
Thanks for heads up!
So, if I update the table in app1, app2 will get the updated data in real time?