The Ipad is a consumer electronics device, with a build quality near the top of what is out there. But it can obviously fail so you would never use it for anything which is critical, without a backup.
I think the airlines use them for airway manuals, which are a huge waste of space and weight, not to mention a
huge ground cost due to the manpower to stuff the updates into them every AIRAC cycle. I knew a man whose wife worked full time at an airline, stuffing in the updates
They probably also use them for reference manuals. But they will carry a second one, for sure.
There is a residual issue with electronic displays of any sort: if you have two the same, say two Ipads, then a bug in the display software could well crash when displaying a particular PDF page, on
both units
Any software developer will be familiar with this
So carrying two Ipads only protects against a hardware failure. With the poor quality of Jeppesen software, this would concern me quite a bit. I saw the new JV4 the other day... it has a feature whereby you hover over an airport in the airport list, a popup appears showing that airport's details. Unfortunately the popup remains in place even when you move the cursor away from the airport name and just covers a part of the screen
There appears to be no config for disabling the popup.