Relatively easy. Here's what you do:
- Create an external data source pointing to the new file.
- Copy the table to the new file.
- On the Graph in the original file, point the table occurrences (TOs) for the relocated table to the new file. (Just double-click on each one and choose the data source from the pull-down menu.)
Remember that FileMaker always works through TOs. If you direct the TO to a different source, all the relationships and script references still are preserved, because they point to the TO, not to its source.
Make a copy of your file to create FileB.
Strip out whatever you don't need from FileB ( optional )
In FileA choose Manage > External Data Sources
Add your FileB there
Now in Manage Database double click your tables that must come from FileB and point them to your External Data Source
Later when you upload FileB to the server just create a new External data source pointing to the server.
Remove the old reference ( just in case it will want to grab the local FileB's data.
There is more to splitting files. You must take care the accounts, passwords and preferably privilege sets match.
Learn how to handle re-login, etc...