Three mergers and acquisitions powerhouses — Cleary Gottlieb Steen & Hamilton LLP, Wachtell Lipton Rosen & Katz and Sullivan & Cromwell LLP — worked on CIT Group Inc.’s proposed $3.4 billion acquisition of OneWest Bank parent IMB Holdco LLC.

While Wachtell represented CIT, the commercial lender led by John Thain, and Cleary represented OneWest, Sullivan & Cromwell provided additional regulatory counsel for both parties because of its banking expertise.

S&C’s Rodgin Cohen said in a telephone interview yesterday that the dual representation was “unusual, but we know both organizations and in a transaction like this, the two parties have a common interest to get the deal approved as soon as possible by the regulators,” including the Federal Reserve and the Comptroller of the Currency.

The deal was previewed with the Federal Reserve, Cohen said, because “the Fed has made it very clear that any deal of significance is to be discussed before it is announced.” He added that the Federal Reserve doesn’t “give any pre- clearance” and so approval, which rests on issues like the financial stability of the combined companies, “could take from six to nine months.”

The Wachtell team representing CIT were corporate partners Edward Herlihy and David Karp, regulatory counsel Richard Kim, executive compensation and benefits partner Jeannemarie O’Brien and tax partner Joshua Holmes.

On the Cleary team representing IMB and OneWest were M&A partners Christopher Austin and Benet O’Reilly, along with banking regulatory partner Katherine Mooney Carroll, employee benefits partner Arthur Kohn and tax partner Yaron Reich.

S&C partners Mitchell Eitel and Camille Orme worked on the deal in addition to Cohen.

Under the terms of the deal, IMB Holdco shareholders will receive $2 billion in cash and 31.3 million shares of CIT common stock, New York-based CIT said yesterday in a statement.