I run 2 canisters on my larger tanks - 55 and up. Each one is capable of filtering the tank on their own. Some will say that that is unnecessary, and they would be correct....from a water quality standpoint. However, I ask more of my filtration systems than merely maintaining water quality. My filtration systems also have to provide a circular flow in the tank, which is achieved by setting them up in opposition to one another. One pushes water along the front of the tank towards the seconds intake, and the second pushes water back along the back of the tank, towards the first's intake. In my experience the fish prefer such circulation. The other advantage of this is waste collection. Since setting up my tanks this way, I never see waste on the sand - keeps the tanks spotless. Another benefit, which boxercrazy mentioned, is that the filters don't need to be cleaned very often. The smaller the system, the more often it needs to be cleaned. I don't clean mine more than twice a year. And lastly, is redundancy, though that's really an afterthought for me. People say to run 2 heaters, that way in case one fails there is a backup. I don't see how this principle does not extend to filters.
Is a 2 canister setup ideal for every tank? No, of course not. Can many tanks benefit from such a setup? Yes, absolutely. It's up to each fish keeper to determine what's best for them, based on their needs and what they want to get out of it.