If you establish an external data source link to the other file, the Perform Script script step can perform a script found in another file. There's a drop down in the dialog that lets you select either another file already listed or to use Add FileMaker Data Source to link to and select a file from one not already so linked.
Perform Script on Server has the same drop down menu to use for the same purpose. (But use PSOS with care as it performs from the context of a host session and not your current client session. Current record, current layout, current found set all are not accessible to a script performed via PSOS.