I have to disagree on the risk being unacceptable, John. Sure, there is a risk there, but it is an acceptable one, provided the back-up has a good amount of trigger time on heavy calibres against heavy animals (pick me, pick me!).
One thing I will concede, though, is that to be a good back-up, you really do need lots of experience with the heavy stuff, so that it is second nature. You can't hand a back-up rifle to a fellow who's shot a few pigs and goats with a .243 and feel too safe and secure.