Combine: update values each other

As you can see when you attempt to add your currently-commented second sink, you’ll end up with a circular dependency between the inches and centimeters. Instead, I’d suggest you store one value and use a custom binding for the other:

struct Centimeters {
    var value: Double
}

class SizeValueModel: ObservableObject {
    @Published var centimeters: Centimeters
    
    var inchesBinding : Binding<Double> {
        .init {
            self.centimeters.value / 2.54
        } set: {
            self.centimeters.value = $0 * 2.54
        }

    }
    
    init() {
        self.centimeters = Centimeters(value: 1.0)
    }
}

struct ContentView: View {
    @StateObject var model = SizeValueModel()
    var body: some View {
        Slider(value: $model.centimeters.value, in: 0...100, label: {
            Text("\(model.centimeters.value)")
        })
        Slider(value: model.inchesBinding, in: 0...39.3701, label: {
            Text("\(model.inchesBinding.wrappedValue)")
        })
    }
}