Yes you can get stereo using the Aux connections leads at the rear of the dash monitor. But as you assume correctly this won't achieve a visual input to the front monitor.
The Aux input will not be shown as a selection until this lead has been connected it can then be sourced via the audio settings. In theory this would then allow you to have stereo sound.

I suppose it all depends on what you hope to achieve, knowing that the dash monitor cannot be used in motion (without adaption).
If you just wish to ensure stereo sound for those watching the rear monitors then this will achieve your goal. It will also ensure your Satnav screen can be constantly accessed without the constant warning signs.

If you wish I can exam my installation further and see where the connections are made?

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