Duplex - 1 mailbox/1 address

[Chantelle Lewis]

I just closed on a house that used to be a single family, but was converted to a duplex. There is only 1 mailbox and 1 address. 

We attempted to have a second address assigned to the property but ran into some issues so We would like to add a second mailbox and add unit A and unit B.
Does anyone have experience with this? Is it a simple as just purchasing the mailbox? Do we need to notify USPS?