She Still Sings It with the Band
Get all possible Double Fab ratings for every song with 2 vocal parts.
If you have a partner, you should go with Davey the Rebel's solution. If not, heres how to do it alone. Two playthroughs required:
1st off you need a PC or any other recording device. I used a PC with a freeware recording program called Free MP3 Sound Recorder: http://www.nbxsoft.com/
Now you will need your two mics. Connect one to your xbox and the other to your PC(not nessesary if using a different recording device)
Now play each song on Harmonies/Easy doing only the main/blue part. Sing into both mics after you hit record on your recorder. I synced by hitting the record button just as the vocals started.
If by the end of the song you scored 100%, your all good.
Now connect your 2nd mic to the xbox, and place your 1st mic infront of your recorder/PC speakers. Play the song again, this time letting your previous recording do the blue part, while you do the brown/orange part on the second mic.
I got all the harmonies on 13 of 14 songs this way. By the time of writing Im still struggeling with "I've Got A Feeling", but thats because of a complete lack of skill lol
EDIT: Got the achievement now. Turns out Free MP3 Sound Recorder isnt free after all and stops working after about 15 recordings. Use Free MP3 Recorder ( http://www.tucows.com/preview/506678