Two `async let` do not run the calculation in parallel

The following program should run the function longCalculation twice in parallel if the button is pressed. But the function is called twice in sequence instead.

struct ContentView: View {
    @State private var result: Int = 0

    var body: some View {
        VStack {
            Text("Result: \(result)")
            Button {
                async {
                    async let result1 = longCalculation()
                    async let result2 = longCalculation(
                    self.result = await result1 + result2
                    print("[end] action / is on main: \(Thread.current.isMainThread)")
                }
            } label: {
                Text("Button")
            }
        }
    }
}

fileprivate func longCalculation() async -> Int {
    print("[start] longCalculation / is on main thread: \(Thread.current.isMainThread)")
    var c = 0
    for _ in 0...10000000 {
        c += Int.random(in: 0...100)
        c = c % 13
    }
    print("[end] longCalculation / is on main thread: \(Thread.current.isMainThread)")
    return c
}

According to the output the calls are still in sequence. Why does this happen? The two async let assignments should create two child tasks. Did I miss something?

> [start] longCalculation / is on main thread: false
> [end] longCalculation / is on main thread: false
> [start] longCalculation / is on main thread: false
> [end] longCalculation / is on main thread: false
> [end] action / is on main: true