Multiple WinRT CaptureElements

The CaptureElement XAML control is a cool and easy technique to preview the camera on a device in your application's canvas without having to invoke the CameraCaptureUI dialog.

But... adding more than one doesn't seem to work.

Trying this:

var _MediaCapture = new MediaCapture();
await _MediaCapture.InitializeAsync();
Cap1.Source = _MediaCapture;
Cap2.Source = _MediaCapture;
await _MediaCapture.StartPreviewAsync();

Results in this on the second 'Source' line:

{"A method was called at an unexpected time. (Exception from HRESULT: 0x8000000E)"}

Is there a technique that allows more than one CaptureElement on a single XAML canvas?