Two Family homes are essentially in 2 categories. Will it be Owner Occupied or an Investment. Some properties can be both.
As an investment, ROI is important. That would include the rent income and the expenses. You also need to consider expected appreciation (future growth in value).
As an "Owner Occupied" property, the Buyer's needs and desires will be an important consideration. The features of one of the units must offer the appropriate creature features.
Be prepared to demonstrate the Rent History and Expenses. A smart buyer will want to see that.
I have 4 Residential Investment Properties, so I have "been there, done that" for myself as well as clients.
Let me know if I can help further.