QFT!
There is absolutely no need for redundancy if you are properly cleaning your filter. If cleaning your filter is leading to a mini cycle, the problem is not the filter, its the person cleaning it. You don't lose measurable amounts of bacteria in the mechanical transfer of cleaning your filter sponge. the only way you will lose enough bacteria to cause some kind of major reaction, is if you did something wrong. (rinsing with tap or chlorinated water, let the media dry out, etc).
You can remove 1/3 of your media without causing any major damage to your system. This is done all the time for seeding new filters and for cycling new tanks. Bacteria colonies can double in size in 24hours. So even if you killed half the bacteria or removed half the media, your system would be back to normal after 24hours (but of course this also means you will need to monitor WP and do a major WC during this period).
As far as the "if one dies, you still have another one" scenario goes, its really not even an issue worth mentioning.
Members here have AC110 (AC500) filters running that are 15+ years old. My oldest one it about 9 years old now. These are some of the most reliable filters on the market. So unless you are not properly caring for your equipment, your chances of having one break on you are slim at best.
Even if you were to buy 2 brand new, the chances of one of them breaking, iare just as good as the chances of both of them breaking. So why not run 3 AC50s, or 4 AC30s?
All these "what if" scenarios are not only unlikely, but irrelevant. Take care of your equipment, clean them properly, and you will be replacing your tank before you replace your filter.