You can do this with a guest account. Just sign in another account, connect 2 instruments of the same kind, and play a Pro Face-Off match. Select to find opponents on live. If no one comes in, the game will start anyway after about 20 seconds. After the song wait the timer on the statistics screen to run out and then select to quit. Search for opponents and repeat. Do this until you get the achievement. It´s not important if you do this alone or if someone plays on the other instrument. Note: play on the same difficulty, stay on the same instrument, and play another song every other time.