The two companies aim for a spring 2011 release for "The Conspirator," which premiered at the Toronto International Film Festival.
The $20 million production directed by Redford was one of the biggest films to enter the festival without a distributor already in place.
"The Conspirator" stars Robin Wright as Mary Surratt, a boarding-house owner tried as an accomplice in the assassination by John Wilkes Booth. James McAvoy plays a Union Civil War hero reluctantly forced to defend her.
The film co-stars Kevin Kline, Evan Rachel Wood, Alexis Bledel, Tom Wilkinson and Justin Long.