When two display devices (eg. a TV and a projector, or two TVs) are connected to the CXR and Twin Display mode is being used, the outputs are mirrored, not independent of each other.
The CXR will therefore 'inspect' at any TV/Projector that is connected and tell the source about the lowest resolution display, ensuring that only up to that resolution is sent. This ensures that an image is able to be displayed on both displays.
For example, if there is a 3D Projector and a 2D TV connected, only 2D will be sent to both displays. You would need to disconnect the 2D TV to allow a 3D image to be sent to the 3D projector.
So, to receive 3D or 4K on both displays, both connected displays must be 3D or 4K compatible, otherwise only the lowest common resolution will be sent to both displays.