Hello I am new to the fish world and also to this forum

i have never owned a canister filter and i would think you only need one of those for a 40 gal. unless you want the other for a spare.hmm you are turning you water over about 6 times per hour that should be plenty of mechanical filtration.

i use one fluval 404 on an 87g if that gives you an idea.

Welcome to the hobby. Previous are probably right about needing only one 404, but if you've already got 2,don't worry about it, you'll probably be wanting another tank before long. Were you warned? Fish are addictive. That said, I use 2 filters on my tanks, no matter what size. I never have to worry about killing my biological filter when cleanng and always have a seasoned filter to start a new tank. On my 50 gal I have an aquaclear 500, emperor 400, and a powerhead with hydro sponge filter ( mostly for current, honest). I have cories, rainbows, raphaels, rasboras, ottos, and loaches in this tank and they are doing well (oops, i forgot the bristlenose). One thing you might want to get or make is a python, it makes water changes sooo much easier.

I use a Fluval 304 on an understocked 29 gallon so a 404 on a 40 is no problem. Get some test kits and test often, espeically for nitrates. If those are constantly climbing high (above say 20 ppm) even with weekly water changes then maybe extra filtration will be needed. But the fish you are thinking will be fine with just the one filter and the current won't be a problem. Make sure to get schooling fish in groups of at least 5 (so five cory cats, five barbs, five tetras etc.. and all of the same species of course).