Do you have the Arduino Uno board ?
The RX (pin 0) and TX (pin 1) are always connected to the usb-serial.
If you connect something else to pin 0 and pin 1, the signals of the usb-serial are overriden, but they are still connected. That is regardless if the Arduino board is powered with the usb plug or not. So you can keep on using the usb to power the Arduino.