Applying USB to UART and RS-485 to UART to the single MCU UART port
An alternative solution: The FT232BM has a pin named #PWREN: Goes Low after the device is configured via USB, then high during USB suspend. Can be used to control power to external logic (ex. using a P-Channel Logic Level MOSFET switch). Maybe turn-off the RS485 device.

The PWREN# signal will be high (power = off) under the following conditions:

· The FT232BM / FT245BM device is in reset

OR

·The FT232BM / FT245BM device is not yet configured by USB enumeration, or the USB enumeration sequence has failed

OR

·The host PC system is in the USB suspend state