Hi Clive,
Just read that post front to back very interesting indeed, thanks for that mate I missed that one when browsing through this sites archives, well worth reading.
Personally I like the idea of having two filters as a back up system, similar in principle to why people back up computer hard drives. If one fails then you don't loose everything, the tank is still effectivly filtered whilst in its reduced capacity (assuming both filters have adequate capacity for the tank they are used on in there own right) and you will still have enough of a bacteria colony left to re-seed the failed filter once its restarted or replaced if the need arises. Now correct me if I am wrong but it is my understanding that once the oxygen is depleted within a filter the bacteria die off fairly quickly possibly even within a couple of hours serious damage could occur to the bacteria colony. Now if you only had one tank which was quite heavily stocked you could end up in a lot of trouble quite quickly as you would be effectivly cycling a tank from scratch again should the filter fail and it went unnoticed for a few days. However if you have multiple tanks and filters available to you its not nearly as much of a problem to deal with.
Therefore to add my pennyworth of advise into the melting pot here I would suggest that if you only have one tank always go for multiple filters for safteys sake if you can. But if you have multiple tanks and filters at your disposal then you can safely consider putting all your eggs (or bacteria in this case) in one basket. The old addage 'the more the merrier' still applies though, IMO.
