zoomkat:
If you are using the hardware serial (pins 0, 1) to communicate with the PC you can't also use it the connect with another device.
Bad information that is becoming a forum urban legend, probably because many don't understand the TTL operations. The arduino can send tx to a large number of rx, and the arduino can receive rx from a large numbet of tx if each tx has a properly aligned diode in the tx line.
But Serial protocol was not intended for doing a multiple nodes network, was it? CANBUS or MODBUS, or I2C are more suitable for that task (I'm talking about the protocol, not the physical layer).
To the OP, If you "other device" supports I2C and it is close to your arduino, I would recommend using RS232 between Arduino and PC, and I2C between your other device and Arduino.