In short, alot of new investors in multi families think you just take you rental income and subtract your basic expenses PITI. prin. interest, tax and insurance and do not take anything else into account. There are alot more things going on. Granted on smaller 2 families they don't play as big a roll compared to larger multi families but you should definitely learn these things now.
While you may have several months of just these basic expenses, hell I have had over a year with just these expenses, you have to look at the big picture esp. if you are in for the long haul. There are expenses that are bound to come up. It's not a matter of IF but WHEN. If you can properly align your budget and account for these things from the gecko you are much more likely to succeed in your investing. The less surprises theh better!!
Don't forget you will also get some great tax deductions and depreciation that will hopefully put money back in your pocket come year end.
regards,
chris