I'm pretty sure it got fixed, last time it worked was Fifa 2012 I think.
You need 3 controllers
1 On the dashboard , Sign into your main profile with controller 1.
2 Turn on controller 2 and click "Play
Fifa 12" whilst on the dashboard still.
3 When
Fifa 12 loads, press start with controller 2. It will give an error message and you select the "Continue without saving".
4 Try to enter the Ultimate Team with the controller 2, do not select any profile. It will give error. Repeat this step again.
After that enter the ultimate team with controller 1. It will give error "Appears that the ultimate team is not available ",
then click "Ultimate Team" again with controller 1
In the menu of ultimate control team with controller three go to Play Match, Tournament, select the tournament, and press (A) to enter the tournament, will give an error, then you press (A) with a controller 1.
7 In the squad press Start to search for game with controller 2.
8. The glitch will only work if you are playing away. Once in game quickly enter as a guest by pressing (Y) with controller 3.
9. Once signed Ready up by pressing (A) with the controller 1.
10.Once the game loads you should be on your opponents team while you team is being controlled by a legendary CPU.
11. Do change the controls whilst in game
12. When you play another game, Continue from step 7