Kerry makes a good point that financing a multi-dwelling, single parcel property can be challenging. I ran into this same issue with a property I bought that had 4 dwelling units on the parcel. First, check the zoning for the parcel. Is it conforming, meaning the multi-dwelling units are allowed. This can be done by checking with your local government's planning dept. (city, town or county). If it is an allowable use, you will need to hunt for a local bank that is willing to lend on the property. This is the tough part but not insurmountable. I went through 5 or 6 banks before finding one that was willing to do a portfolio loan on my parcel.
One other item to consider is the value of the property. You mention a cap rate, but with the parcel having 2 units, the appraised value will be determined based on the sales price of similar properties in your area, not the net operating income (NOI) the property can generate. The bank will be looking at the NOI for purposes of debt coverage, but the appraiser will not take this into account since the property is <5 units.
Kerry's counter for better owner financing is the best way to go. I'd do further due diligence because when the balloon term arrives, if the owner is not willing to continue, you will need a backup lending solution. Another way to counter the owner is to ask how much he/she wants to make each month on the payment. You can back into this figure with terms which could avoid getting hung up on single components of the lending calculation (rate, DP, amortization, etc.).