Depending on what you want from your carpc, just one tablet might be enough. There are Youtube videos reporting tablets as the HU screen. There are apps such as AutoMate, car dash android-Car, Android Auto which are quite close to what I would like to see as a HU screen. To automate routines I use MacroDroid.
With tablet as HU screen it is important to have light sensor in the tablet.
With tablet as HU screen it is important to have light sensor in the tablet.