DANBURY, Conn. (AP) _ Union Carbide Corp. will reorganize its top management, creating two ''co- presidents'' to run the giant chemicals company, according to a published report.

The reorganization is expected to include naming Alec Flamm, now president and chief operating officer, to a new position of vice chair man, the News- Times of Danbury reported today.

Replacing Flamm and holding the concentration of management authority as co-presidents will be Robert D. Kennedy and Heinn F. Tomfohrde III, the newspaper reported.

The reorganization follows steps taken last week by Carbide's board of directors to amend corporate bylaws in order to make a hostile takeover more difficult and less lucrative for any potential buyer.

In addition, the News-Times reported that Union Carbide is expected to offer its employees an early retirement program aimed at reducing its worldwide employment levels by about 15 percent.

Carbide employs about 2,900 people at its corporate headquarters, about 48,400 in its domestic operations, and another 46,700 people worldwide.

The early retirements could be funded by the $764 million in surplus pension funds without having an impact on third-quarter earnings, according to C. Howard Nichol, vice president of research at Advest Inc. in Hartford. Without the surplus, Union Carbide would have to set aside reserves to cover the program, he said.

The early retirement program would affect about 435 people at the corporate headquarters here, most of whom are expected to be middle management employees who are close to retirement age but have not yet accumulated enough ''points'' under the company's retirement plan to seek early retirement.

The News Times, quoting sources, said the reorganization was announced to divisional presidents Friday, and that divisional vice presidents were told of the changes on Monday.

The reorganization is expected to name Kennedy and Tomfohrde to oversee the operations of Union Carbide's 17 different divisions and wide range of subsidiaries, departments and companies now split between five executive vice presidents.

Kennedy will be a co-president with responsibility for the company's industrial operations while Tomfohrde will be a co-president with responsibility for the chemical operations, the newspaper said.

Kennedy was named an executive vice president in 1982, and Tomfohrde was named an executive vice president and head of the engineering and technology services group in 1983.

The remaining three executive vice presidents are John H. Field, James M. Rehfield and Elio E. Tarika, all of whom are expected to report to Flamm on ''special assignment,'' the newspaper reported.

J. Clayton Stephenson, executive vice president and chief financial officer since 1982, is not expected to be affected by the reorganization, the newspaper said.