constraints for child view controllers in scroll view not working as expected

I have two child view controllers in the content view of a scroll view. the first one is constrained to the top of the contentview, the second is constrained to the top of the first view controller.

The problem: the second view controller is not appearing as expected based on constraints (second view controller is colored box at the top, first is box below that).

Is it a problem with the scroll view or my constraints?

scrollview setup:

• content view constrained to scrollview not frame layout
• scroll view constrained to superview safe area
• content view width equal to superview

here's my constraints code

 override func viewDidLoad() {        super.viewDidLoad()        contentView.addSubview(planningSessionVC.view)        contentView.addSubview(attendeesVC.view)        scrollViewRightHidden = scrollView.rightAnchor.constraint(equalTo: view.leftAnchor, constant: 0)        scrollViewRight = scrollView.rightAnchor.constraint(equalTo: view.rightAnchor)        scrollViewLeft = scrollView.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 0)        planningSessionTop = planningSessionVC.view.topAnchor.constraint(equalTo: contentView.topAnchor, constant: 60)        NSLayoutConstraint.activate([            planningSessionVC.view.leftAnchor.constraint(equalTo: contentView.leftAnchor, constant: 15),            planningSessionVC.view.rightAnchor.constraint(equalTo: contentView.rightAnchor, constant: -15),            planningSessionTop!,            scrollView.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 1.0)            ])        view.layoutIfNeeded()        NSLayoutConstraint.activate([        attendeesVC.view.leftAnchor.constraint(equalTo: contentView.leftAnchor, constant: 15),        attendeesVC.view.rightAnchor.constraint(equalTo: contentView.rightAnchor, constant: -15),        attendeesVC.view.topAnchor.constraint(equalTo: planningSessionVC.view.bottomAnchor, constant: 0),        attendeesVC.view.heightAnchor.constraint(equalToConstant: 50),        attendeesVC.view.bottomAnchor.constraint(equalTo: contentView.bottomAnchor, constant: 20)        ])        view.layoutIfNeeded()    }