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Sounds like you need to learn more about how FileMaker works--there are books and tutorials available.
What you describe as a "secondary database" is just another table you can define in the same file. You can then link the tables in relationships, to link a given customer record in the customer table to all their invoice records in the invoice table to give one example.
With two related tables, you can use a portal to display related data in a scrolling window. There are also ways to set up buttons and scripts that find the related records and display them in a layout for review and/or printing.