One of them know exactly what happened that day.
If mother did something to kids, and father was aware and helped cover, up her crime, he is also legally responsible.
If she does a bolt unexpectedly, he is left holding the bag and answering all the luring questions.
Her whereabouts are unknown immediately. He is confused and doesn't know what happened to her or her reason for bolting. The pack they made to cover crime is at risk of exposure, so he continues to keep the story line going. She has answered all the questions she can, and knows talking too much puts you at risk for slipping up.
So 2 months later, he hasn't been able to talk to her, to revalidate the story they concocted, to validate her silence and loyalty to the pack, but he realizes in that pack and cover up, he's just as guilty, and the heat is on from Joe Public, and the attention is on him. She's scott free from finger pointing while hiding.
Will they come out together and state they renewed their relationship, and are fighting to keep baby, and work through this loss together?
One of these partners could have murdered alone, but I honestly think hiding of the bodies had to be done by both, she wouldn't be able to physically do it unless over a cliff into tidal bore.
If father did it, why would mother keep quiet? She would do so if "Stockholm syndrome", or she was part of crime.
If she was incapacitated in a facility, would her accusations/words be inadmissible if she was declared incompetent? Are LE waiting for a clearance by an MD to pursue further questioning ?
Bolting could clearly be part of fight or flight, possible mental breakdown, fear of something greater? But staying silent this long is looking very bad for her. Even if those kids are genuinely missing, her silence is very strangely unusual