I have to think CW was trying to dispose of all 3 bodies at the same time....
He would torture SW by showing her the dead bodies of her little girls and make her realize he was going to murder her, too.
Then he has 3 bodies and less than 2 hours of darkness to get them out of the house and hide them.
The isolated, remote oil well tank is a cunning idea.
(I'd love to see if he has computer searches for hiding decomposition), and it strikes me as a premeditated, researched idea, not a sudden flash of inspiration. And he could easily scout out the most remote, most ignored one within a reasonable drive.
Then he has 3 bodies to dispose of. LE tells us that the tank was mostly full. Was it too full for all 3 bodies? Did he have to made a difficult choice in the predawn dark out there? Was it better to put the two small ones in there or her body? Tick, tick, tick the clock is ticking and dawn is coming.
So he decides to put the two small ones in there and then has to quickly dig a shallow grave nearby for SW. Did he intend to return in the dark and place her into a second oil well tank?
Hopefully we will know, soon.