Def Jam Rapstar
51 Achievements
It Takes Two
Perform 30 Battle or Duet performances in Party mode
7.1%
How to unlock the It Takes Two achievement in Def Jam Rapstar - Definitive Guide
TheCaptain388
I recommend doing this after completing all of the other trophies which require two microphones (‘Playa President’, ‘Towering Over The Competition’, ‘Close Call’, ‘No Competition’, ‘We Are The Best’ & ‘Rhymes Are On Time’). After completing those, you should have about 20 performances left to do. Simply plug in the second mic and set up a long playlist with a short song like “We Fly High” or “Rockwilder”. Just set the microphones down and let them play while pressing 
twice at the end of each song. You cannot fail a song, so no matter how bad you do it’ll still count as a performance. You can also check your progress towards this trophy in the Extras, Player Stats section at the main menu.
ChrisD0811
By the time all other dual-mic achievements are done, you should be at 7 to 10 Battle or Duet Performances total. Create a setlist of "We Fly High" repeatedly, as it is the shortest song in the game. You can make the setlist equal to the number of performances needed, and you can check that number by going into "Extras" and "Player Stats" from the main menu. Because there is no fail, you can simply press 
twice to start the next song each time, and walk away and do something else for a couple minutes. The achievement will pop at the results screen for dual-mic song number 30.