Backup batteries are pretty cheap...why wouldn't you install them for each display (I have an SV-1000T and a D6, both backed up with their own batteries)?
If you were asking me, it was simple - If I ever get to the point where the Dynon screens are operating on backup power in flight, I don't need the other screen.
My airplane is set up for IFR ops - if I'm IFR and electrically normal, then I have my IFR navigator (which does NOT have it's own backup) online and running. If my alternator dies, I switch on my backup alternator and continue. If the SECOND alternator dies, I now have about 45 minutes of aircraft battery reserve running my essential bus and IFR capability to get on a runway, or at least under the cloud deck. After that point all my electrons run out and I lose engine power, since I'm electrically dependent for both fuel and spark. At that point I'm a glider, and a VFR one - but a single backup battery on the primary screen in front of me provides me with GPS mapping and wind direction as well as synthetic vision for the glide down to a, hopefully good, landing within glide range. There's no scenario I can build in my head that requires a backup battery for the second screen that has not already been accounted for another way. By the time the backup battery is in use, I'm already three failures deep on a single flight and planning for that, or further down the chain from that, is simply futile in the Part 91 world.
As was mentioned earlier, I do power up my primary screen on its battery on every start, and let it boot while getting seatbelts on and cockpit arranged for start, so I have engine instruments available without a brown-out on the battery voltage. It also allows me to verify the function of the backup battery on every start.
