That's a little fuzzy. Do you want a list of multiple contacts for a given transaction or multiple transactions for a given contact?
Either way, if you can define a relationship in Manage | Database | Relationships to link the two tables, you can then use a portal to display the multiple list of matching records. You can look up "portal" FileMaker help to learn more.
That should help. I was wondering if there was a different way but I guess not.
You may also be able to set up a summary report that lists your "multiple" entries grouped under sub-headers--which can show data from the related table that all the indivicual entries match to.