Two of America's biggest pension funds just gave Bank of America a thumbs down

The chorus of opposition against Brian Moynihan's dual role as chief executive and board chairman of Bank of America is growing louder.

Two of America's biggest public pension funds - the California Public Employees' Retirement System (Calpers) and the California State Teachers' Retirement System (Calstrs) - sent a joint letter on Monday to the bank's lead director, Jack Bovender.

The two funds wrote that sitting as both CEO and chairman of the No. 2 U.S. bank by assets created a conflict of interest.

The funds, which together hold less than 1 percent of total shares outstanding in the bank, wrote that since Moynihan was appointed CEO, the bank has underperformed and that it needs stronger, more independent oversight.

"We believe the Board's rationale for making this change is fundamentally flawed and we disagree with many assertions made in the Special Meeting proxy," the funds wrote. They also said the company has never provided a valid business rationale for combining the roles.

Shareholders will vote whether to allow Moynihan's dual role to continue during a September 22 special shareholder meeting.

Bank of America was not available for comment outside regular U.S. business hours.

Calpers and Calstrs add to a group of critics including CLSA bank analyst Mike Mayo who have decried the Bank of America board of directors' unilateral decision to allow CEO Brian Moynihan to add the role of chairman back in October.

Bank of America had combined the roles of chairman and chief executive until 2009, when shareholders voted to strip then chief executive Ken Lewis of his chairman title. Investors had objected to his decision to acquire Merrill Lynch at the peak of the financial crisis.