The second vote is to be held two weeks after full results are declared.
Berewa had been considered the favourite to emerge as the African country's new president after the first round of voting on August 11, the first election since UN peacekeepers left in 2005.
However, his task became harder after the man who finished third in the first round, Charles Margai, threw his support behind Koroma.
Margai broke away from the SLPP in 2005 after losing a leadership contest to Berewa and although he trailed the two main candidates by a long way winning only 15 per cent of the vote, his support is seen as crucial in the run-off.
Theophilus Gbenda, spokesman for Margai's People's Movement for Democratic Change, said Margai was looking to move past tribal and regional divisions to the "common good of the country".
Berewa dismissed the announcement saying "Margai's decision to ally with the APC has not hampered my party's hope to win the presidential election if there is a second round.
"It is the democratic right for the PMDC candidate to make his choice as to whatever party he would want his party to ally with."
Officials from both the APC and PMDC did not immediately comment but said they would make a joint statement later.
Meanwhile West African nations warned parties in Sierra Leone not to actively stir up unrest while waiting for the final election results to be declared.
"I hope the people of Sierra Leone can have this election without violence. All that is needed is one act of violence to spark another civil war."
Tom Dougherty, Atlanta, USA
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Mohamed Ibn Chambas, the commission chief of the Economic Community of West African States (Ecowas) warned against actions that would "overheat the political atmosphere as we await the verdict of the people".
"All must desist from any acts of commission or omission that are likely to heighten tensions or destabilise the political atmosphere at this critical juncture in the country's history," he said.
By law the run-off vote has to be held within two weeks of final results being declared.
The new president will succeed Ahmad Tejan Kabbah, the outgoing president has completed his two five-year terms, the maximum allowed by the country's constitution.
The August 11 polls were also to elect a new legislature, and results so far suggest the APC controls 21 of the 112 parliamentary seats up for grabs, followed by the SLPP with 10 and the PMDC with four.
The vote was only the second since Sierra Leone emerged from a brutal ten-year war and the first after the departure of about 17,500 United Nations peacekeepers in 2005.