How can I view 2 camera views simultaneously?

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In most cases application uses only one camera in the same time. So all OpenCV examples added only one view in the window layout and CvCameraViewListener/CvCameraViewListener2 interface implementation to Activity class. It simplifies examples and helps most developers to start easier.

In your case you need to add two views to application layout (vertical or horizontal, as you want) and add two fields to Activity, that implements CvCameraViewListener/CvCameraViewListener2 for both cameras. Separate view components in layout, separate CvCameraViewListener/CvCameraViewListener2 objects for view events handling.

You need to create layout with two JavaCameraViews with different camera indices and call setCvCameraViewListener to both of them. I recommend you to use two separate CvCameraViewListener/CvCameraViewListener2 implementations for cameras to eliminate conflicts. Also, The opportunity to work with both cameras in the same time depends on Android camera hardware and software stack. I do not sure that it works with all double-camera devices.