Separate mail boxes for new duplex

[Christopher Cole]

I'm closing on a house that used to be a single family, but was converted to be a duplex. There is still only 1 mailbox, however.  It seems previously all mail went to the same box, and then the tenants split it among themselves.

I'd like to add a second mailbox and have them be Unit A and Unit B or something like that. There doesn't seem to be much information about how to do this with USPS though. Does anyone have experience with this?