Placing both a P.O. box and a street address on the same mailing envelope is not proper. The U.S. Postal Service uses the address listed on the envelope as guidance for delivery. Since it can only deliver to one address, two different locations on the same envelope will not give the carrier appropriate direction.
The U.S. Postal Service has strict guidelines for how to address an envelope properly. If both a P.O. box and a physical street address is written on the same envelope, that envelope will provide conflicting information. The sorter will not be able to identify which of the two addresses is the desired location of the envelope. By default, the sorter will attempt to use the listed zip codes for clarification. In the event that the zip code is the same for both the physical address and the P.O. box, the envelope will be directed to the P.O. box.