What are the requirements for backup? Must have two iPads onboard? Preflight must include checking battery status? Does the iPad have to belong to the company and is it subject to a maintenance regime?
Backup would be paper plates. No need to check batteries if you hard-wire it (which is pretty straightforward). No need for it to be subject to a maintenance regime, as long as it has an up-to-date database, there is nothing to maintain! Oh, and the latest ones have serial ports, so you can interface directly with your GPS for moving-map functionality...