I dunno that I buy into the whole dishes in a dishwasher analogy... Water passing through a filter in a single pass doesn't really come out completely clean, especially in the ammonia and nitrite side of things. With other filter systems (oil, fuel, industrial, etc) you see single pass as well as multi-pass performance specs.
Basically, the more passes the water makes through the filter the cleaner it will be. Adding the AC
filter to the output of the canister will somewhat help with this. The bigger thing is the colonization of your beneficial bacteria. By providing more area for the BB to colonize you'll enable them to handle a larger bioload more effectively. Granted, either way it doesn't really matter if the AC
is self powered or fed by the other filter.