I'm trying to run ARWorldTracking Session and ARFaceTracking Session at the same time on iPhoneX,
 but the first running session stopped after the later session begun to run.

Is is impossible to implement?
This is my ViewController.swift code.

```
import UIKit
import ARKit

class ViewController: UIViewController, ARSCNViewDelegate, 
ARSessionDelegate {
    @IBOutlet weak var frontView: ARSCNView!
    @IBOutlet weak var backView: ARSCNView!
    override func viewDidLoad() {
       super.viewDidLoad()
       startTracking()
    }

   func startTracking() {
       let backConfiguration = ARWorldTrackingConfiguration()
       backView.session.run(backConfiguration)

       let frontConfiguration = ARFaceTrackingConfiguration()
       frontConfiguration.isLightEstimationEnabled = true      
       frontView.session.run(frontConfiguration)
  }
}

```