You've posted your question in exactly the right place.
There are many ways to do this in FileMaker.
You can set up a button for each file that uses the Open File step to open the other file.
You can setup a drop down list that lists all the files with a srcript trigger that performs a script to open the selected file.
You can use FileMaker advanced to add a custom menu that lists the other files and selecting an option from this menu use Open File to open that file.
You can also use Perform Script in each of these scenarios to perform a script in that file instead of using Open File. That option enables you to both open the file and do some initial setup such as selecting a specific layout, performing a find and/or sorting the records.