Financing Questions on an Atypical Duplex

[Sam Rust]

I'm under contract for $340k on a piece of property that has two separate houses and an unattached garage. The houses are both rented up with a total gross rent of $2500 per month. I reached out to my mortgage broker who has done my last couple of SFR, and he is telling me that I need to put 25% down if it is a true duplex. This raises a couple of questions:

How do I determine if it is a "true duplex"? I looked it up on the county site and it lists both dwellings as residential. In talking to the owner, the buildings are both legal and are grandfathered into the zoning type.

Do I indeed have to put 25% down on a true duplex?

Is it possible to use the rents as a case for a lower percentage down? 

Thanks!