Marketo custom objects - unique identifiers for dedupe?

Hi all

I'm creating a Marketo custom object and think I'm missing something obvious...

I have a one-to-many relationship setup. Basically, there's an ID field in Salesforce on our Person Accounts that is the ID of another account. It represents the travel agency that a person last booked with.

So I'm creating a custom object to hold further information around that agency. Fields on the custom object would be something like this:

* Agency ID (link field, dedupe)

* Agency name (string)
* Address (string)
* Phone number (string)

So this all makes sense to me - I link the Agency ID custom object field to the "Last Agency Used" field on Company (since we use Person Accounts).

But here's where I'm struggling to understand how it works: with the current structure, as I understand custom objects, each unique custom object record can only be associated with one person in Marketo. But, each travel agency can make many bookings! 

So if I upload a csv of agencies by ID, then that particular agency won't be associated with ALL of the customers it booked - but instead, simply one. Right? That's not what I want.

But I don't think this should be a "many to many" relationship since one person can only have one agency that they last used. But, that agency can have many customers that last used it. Any ideas?

I understand this probably isn't the ideal setup (you could do this via custom objects in Salesforce more easily) - but long story short - since our Marketo instance holds both consumers and travel agents, we can't associate those custom objects with both of them.