i would consider the case you described as being double jeopardy unless for some reason you believe the two are linked. becuase it sounds like you might have a simple regulator in and a back pressure regulator out.
what is the likelihood of failure at the same time? and are the failures related?
is the depad device a simple back pressure regulator? or is a control valve with a control loop from a PIT?
is the pad regulator a pilot operated with remote sensing line that is also connected to a common valve that provides the tap to a PIT to a PIC? that could result in a common root cause failure that could impact both the supply and outlet, etc.
are you likely to be filling the tank and using the capacity of the depad regulator when you have the supply failure?
you need to define your credible scenarios and then address them.
