BM previously stated using both embedded audio and aux XLR audio mixed together or on separate tracks was not possible due to hardware limitations on the current model. My assumption is, the audio processor can only handle two channels, like the Hyperdeck Shuttle, and does not have internal mixing capabilities to mix both sources, due to audio processed limitations.
My only insight is 20+ years of experience in audio recording and audio solutions for video recording.
An audio recordrecording need hardware support for multiple track recording, you can not get 4-tracks out of a recorder designed to support two tracks. To merge four channels of audio into two recorded tracks, requires some type of audio mixer processor to do this, an a means to control the level of each input, something the VA lacks.
The PixE has 6 to 8 tracks available, while the Atomos Shogun has 12 tracks available of embedded audio.
The VA was designedvas a simple, low vista monitor recorder, aimed primarily at the DSLR video market, and requests for external audio inputs on the original VA HD, resulted in the additional audio inputs on the newer VA4K.
Cheers