Apartment numbers for the units in my new 2-fam house

[Christopher Cooley]

This is probably an odd question, but I need to find out how the USPS designates the two units in the house I'm buying.  (#1 #2? Upper, lower?)  I'm closing and moving into it in the next few weeks, so I need to be changing my address with the P.O. and other businesses, but I want to be sure the address I give is complete, so that the postal worker can easily separate my mail from my future tenant's mail.  It's no use asking the seller; he just bought it, renovated it, and flipped it, so I'm pretty sure he never received mail at the house.  I've been googling but can't figure this out!  Thanks.