Separate Living with Separate Entrance - with a shared mailbox

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We rent out the upper and lower levels of a single-family home.  The units are separated by a stairwell and permanently locked door.  The house functions like an upper-lower duplex.  One issue that I have not been able to resolve is the "shared mailbox."  I have asked the postal carrier about placing a mailbox next to the current mailbox to separate the tenants' mail, but he said that is not an option.  Since it is a "single-family home" it can only have one mailbox.  Has anyone else come across this issue?  And do you have any suggestions on how to resolve.  

Thanks!

Jodi