If you are going to have your son in one unit and a friend in the other.. I would be more concerned with location then cap rate. In Southern CA.. most income properties rarely make a lot of financial sense but the appreciation for a multi-residential zoned property in a good area is usually excellent...good cap rates are usually not found in areas where you would like to have your child living... I would buy in the best location I could afford.. I would also check with a lender who knows small unit financing.. and be prepared to run a deficit for a bit.. I would contact.. Brian Brady.. @ http://brian-brady.com..
he knows a lot about this type of financing and can offer you the best advice about ways to financially structure the transaction.. Hope this is helpful..