While not as simple as in Access, I haven't found splitting a FileMaker Database all that difficult to do: Convert to Seperation Model
A data separation model is a quite reasonable option. But whether you use that method or not, The time will come when you need to move data from one file to another. (Even data files need updating at times) The import records process can be scripted so that the user need only click a button or can click a button and then select the original file in an open file dialog. Then the script pulls in all the data and updates the next serial value on any fields with auto-entered serial numbers.