A source close to the couple tells PEOPLE: "I don't think this relationship (between Richards and Sheen) is going to work."
Sheen's rep, Stan Rosenfield, had no comment.
Until Wednesday's filing, the pair – who split last spring while Richards was seven months pregnant – seemed to be patching things up. After their daughter Lola was born last June 1 (older sister Sam turns 2 in March), Sheen and Richards had been spending more time together, including a 65th-birthday party for Sheen's father Martin on Aug. 7, where they shared a loving smooch.
Sheen, 40, admitted publicly that he and Richards, 34, were working on a reconciliation. "Things are a whole lot better, and I think with that (comes) hope," he told TV's Extra in an interview broadcast Aug. 22. "We're not ruling anything out."
The actress, who cited irreconcilable differences in her March divorce petition, was also sporting a new diamond on her left ring finger last summer. While it looked strikingly like the 4.3-carat engagement ring Sheen gave her in December 2001, it was actually "a recent gift from Charlie," a source close to the couple said, "showing her how much he wants things to work out."
Sources also claimed that Richards was equally open to a reunion. "She never wanted to get divorced," said one. "She always hoped they could save their marriage."
As matters progressed – or, now it seems, regressed – the two maintained separate residences: he at their Agoura Hills, Calif., home and she at a new house a few miles away.