I think they're both guilty. They both slept through their sons being murdered. Neither child woke up while the other was being murdered? Either each parent murdered a child, or one parent knew the other one was murdering them.
Darin put that sock in the alley. Helped Darlie inflict her stab wounds to be non fatal. I believe their thinking was that they could start fresh with baby Drake.
I also get why Darlie hasn't implicated him by now. During the trial she was still in love with him. And i'm sure he convinced her she'd never be found guilty. Now if she changes her story she looks like a bitter ex trying to throw him under the bus, or someone desperate to get off death row.
I think people have a hard time believing a mother could kill her kids. Even though we see case after case of it on the news.