Dynon Skyview HDX with a backup Dynon D6 .. wish I hadn't fitted the silly useless round things
To be honest, I don't know what our obsession is with back ups to back ups (and I'm guilty of having a back up to my EFIS). I bet your HDX has a back up battery and yet you still have a D6, analogue dials AND all the information you really would need to get you down safely on your SkyDemon.
If I had my time again it would be a Dynon EFIS and an iPad in a Guardian Avionics mount. That's it. There's no need for anything else in a day, VFR machine. If the EFIS goes belly up and the back up battery doesn't help then I can get groundspeed and enough of an altitude guide from SkyDemon to sort everything out.......and even if I lost that the machine isn't going to fall from the sky.
(I bet you wish you hadn't bothered with the autopilot controllers, when it's all so easy to use direct from the HDX )