要实现不同UIStackView的子视图之间的等高约束，可以使用Auto Layout来实现。以下是一个使用代码示例的解决方法：

首先，创建多个UIStackView，并将它们添加到父视图中：

// 创建父视图
let containerView = UIView()
containerView.translatesAutoresizingMaskIntoConstraints = false
view.addSubview(containerView)

// 创建第一个UIStackView
let stackView1 = UIStackView()
stackView1.translatesAutoresizingMaskIntoConstraints = false
stackView1.axis = .vertical
containerView.addSubview(stackView1)

// 创建第二个UIStackView
let stackView2 = UIStackView()
stackView2.translatesAutoresizingMaskIntoConstraints = false
stackView2.axis = .vertical
containerView.addSubview(stackView2)

// 建立父视图和子视图的约束
containerView.topAnchor.constraint(equalTo: view.topAnchor, constant: 20).isActive = true
containerView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 20).isActive = true
containerView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -20).isActive = true
containerView.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: -20).isActive = true

// 建立第一个UIStackView的约束
stackView1.topAnchor.constraint(equalTo: containerView.topAnchor).isActive = true
stackView1.leadingAnchor.constraint(equalTo: containerView.leadingAnchor).isActive = true
stackView1.trailingAnchor.constraint(equalTo: containerView.trailingAnchor).isActive = true

// 建立第二个UIStackView的约束
stackView2.topAnchor.constraint(equalTo: stackView1.bottomAnchor, constant: 20).isActive = true
stackView2.leadingAnchor.constraint(equalTo: containerView.leadingAnchor).isActive = true
stackView2.trailingAnchor.constraint(equalTo: containerView.trailingAnchor).isActive = true
stackView2.bottomAnchor.constraint(equalTo: containerView.bottomAnchor).isActive = true

然后，创建每个UIStackView的子视图，并添加到对应的UIStackView中：

// 创建第一个UIStackView的子视图
let label1 = UILabel()
label1.translatesAutoresizingMaskIntoConstraints = false
label1.text = "Label 1"
stackView1.addArrangedSubview(label1)

let label2 = UILabel()
label2.translatesAutoresizingMaskIntoConstraints = false
label2.text = "Label 2"
stackView1.addArrangedSubview(label2)

// 创建第二个UIStackView的子视图
let button1 = UIButton()
button1.translatesAutoresizingMaskIntoConstraints = false
button1.setTitle("Button 1", for: .normal)
stackView2.addArrangedSubview(button1)

let button2 = UIButton()
button2.translatesAutoresizingMaskIntoConstraints = false
button2.setTitle("Button 2", for: .normal)
stackView2.addArrangedSubview(button2)

这样就实现了不同UIStackView的子视图之间的等高约束。子视图的高度将根据其中最高的子视图自动调整。