I would have hung both of them. I don't believe that "post partum depression" is an excuse.
What Andrea Yates had went well beyond "post-partum depression". She had been diagnosed with post-partum psychosis, and i'd be surprised if she isn't also diagnosed schizophrenic by now. She had also been prescribed some powerful anti-psychotic drugs, which IIRC she had stopped taking at some point prior to the murders. Some kinds of mental illness really make the sufferers unable to perceive reality. Andrea appears to have had such a mental illness. Russell, on the other hand, was apparently fully functional in that he was performing satisfactorily in a full-time professional job. He simply failed to take the common sense actions in response to his wife's obvious complete insanity. Among other things, although she had had severe post-partum depression, and I believe also psychosis, after the next to last child, and doctors had warned the Yates against another pregnancy, he wasted no time in getting her pregnant again AND leaving her alone every day with the even larger group of young children.