Hi Bro
Regards to whether running the canister in series or independent depends on what you want to archeive.
1. If you intend just to turn on 1 canister to save power comsumption then you can chain them up in series. DO NOT turn on both, it does not make sense and waste the electricity. THe pro of running in series will be the 1st canister can have your mechanical filtraion (off cos you can add Bio rings..etc) and the 2nd canister can be your chemical (like carbon).
2. If you run independent, then each canister will need to have a min Mechanical (cotton wool) filtration. You will need to filter the majority of the dirt before hitting the Bio. The pro of running this way will be easy maintenance (can shut down 1 at a time) and also can point the flow in at least 2 direction easier.
Just my 2 cents. Previously I am also running on canister filter.