Walk both units
Compare entrances, stairs, storage, laundry and parking. Check how someone reaches each unit in winter and where bins go. Keep separate notes for each unit so differences in condition or layout are visible.
Map utilities and common areas
Ask what is separately metered and who pays for heating, water, trash and shared electricity today. Request bills where available and compare them against the listing. Verify who handles the lawn and snow, and who has access to the basement or mechanical equipment.
Request the property records
Ask for current leases, service history and any documentation the seller has about permitted use. Verify local rental requirements for the specific address with the city. Have the relevant professionals review anything you plan to rely on before you move forward.
Test your assumptions
List the rents you can document, the expenses you can verify and the items that still need estimates. Consider how a vacancy or a major repair would affect your plan. If you intend to occupy a unit, compare the layout you would live in with the practical demands of managing the other one.