Thin border when using CAShapeLayer as mask for CAShapeLayer

Question

In Swift, I have two semi-transparent circles, both of which are CAShapeLayer. Since they are semi-transparent, any overlap between them becomes visible like so:

Instead, I want them to visually "merge" together. The solution I have tried is to use circle 2 as a mask for circle 1, therefore cutting away the overlap.

This solution is generally working, but I get a thin line on the outside of circle 2:

My question: How can I get rid of the thin, outside line on the right circle? Why is it even there?

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The code is as follows (Xcode playground can be found here):

    private let yPosition: CGFloat = 200
    private let circle1Position: CGFloat = 30
    private let circle2Position: CGFloat = 150
    private let circleDiameter: CGFloat = 200
    private var circleRadius: CGFloat { return self.circleDiameter/2.0 }

    override func loadView() {
        let view = UIView()
        view.backgroundColor = .black

        self.view = view

        let circle1Path = UIBezierPath(
            roundedRect: CGRect(
                x: circle1Position,
                y: yPosition,
                width: circleDiameter,
                height: circleDiameter),
            cornerRadius: self.circleDiameter)

        let circle2Path = UIBezierPath(
            roundedRect: CGRect(
                x: circle2Position,
                y: yPosition,
                width: circleDiameter,
                height: circleDiameter),
            cornerRadius: self.circleDiameter)

        let circle1Layer = CAShapeLayer()
        circle1Layer.path = circle1Path.cgPath
        circle1Layer.fillColor = UIColor.white.withAlphaComponent(0.6).cgColor

        let circle2Layer = CAShapeLayer()
        circle2Layer.path = circle2Path.cgPath
        circle2Layer.fillColor = UIColor.white.withAlphaComponent(0.6).cgColor

        self.view.layer.addSublayer(circle1Layer)
        self.view.layer.addSublayer(circle2Layer)

        //Create a mask from the surrounding rectangle of circle1, and
        //then cut out where it overlaps circle2
        let maskPath = UIBezierPath(rect: CGRect(x: circle1Position, y: yPosition, width: circleDiameter, height: circleDiameter))
        maskPath.append(circle2Path)
        maskPath.usesEvenOddFillRule = true
        maskPath.lineWidth = 0

        let maskLayer = CAShapeLayer()
        maskLayer.path = maskPath.cgPath
        maskLayer.fillColor = UIColor.black.cgColor
        maskLayer.fillRule = kCAFillRuleEvenOdd

        circle1Layer.mask = maskLayer
    }

Answer 1

If both CAShapeLayers have the same alpha value, you could place them inside a new parent CALayer then set the alpha of the parent instead.