I would agree that Multi-Family will narrow down your search on Trulia and in MLS. Two-family would be the classification or duplex and sometimes an agent will advertise it in other ways if they feel it can be a condo conversion. I work for Hammond Residential in the MetroWest area and would be glad to help you with your search if you may be looking outside of the Newton market. It would be helpful for you to talk with a mortgage professional to understand the qualifications of a multi-family loan and obtain a pre-approval letter. I would be glad to recommend some if you aren't working with someone already.
Let me know if I can be of help,