They never stopped!
The have done it onstage at the same time during last year's MBC Gayo Daejun.
And now, the've done it again with the same song!

Bom did it during Oh Yeah @ Big Show 2011

And GD did it too while performing Oh Yeah (without Bom) @ the recent StayG Concert!

And lately, Bom's consistently doing it too during 2NE1's I Am The Best stage performances.
GD also consistenly did the sign during Big Bang's Tonight stage performances during the first half of the year.

Credits: G-BOM baidu