I suspect the reality would be you would also be without the "on board" radios becasue even if these continued to operate you would have no way of knowing what frequency was selected (although I suppose you could stay on the last selected, or if really clever might work out how many turns did what!!! ).
Assuming it is just the screens that are dead....just hold down the frequency change over button for several seconds and it tunes the radio to 121.5.....I believe the radios go to 121.5 in the event of both systems failing completely.
You can try these AHARS or ADC failures in the air by pulling the relevant circuit breakers........
I think that although you can get hung up on scenarios like this, it is far more likely that you will be flying in a conventional aeroplane and loose all nav than one of these types (I know people it has happened to in IMC). It is also worth bearing in mind that the benefits probably outweigh any perceived risks. With the very accurate map, if you lost your engine at night over hostile terrain or in IMC, you *may* be able to navigate to a road or other suitable area and land on that. I simulated EF in IMC over LA in the sim (FAA approved with surround external visuals). Found a road and broke out of the cloud at 500' lined up and landed. Ok it was a sim, but get my point?
