All my German Ram tanks that are not fry/or breeding tanks have multiple filters.
It is like I and M just said.
I don't need all the filters for bio,
but to control my mechanicals properly and have the highest water quality for my ram breeding this is the configuration that works best(still changing more water then most also).
I actually considered moving out the sponges just today, but I know they are the best thing going if you don't mind the look.
I got two 110s on my 75g ram growout!(with air driven sponges and one sponge run by hagen 50 powerhead(old model 50!).
Many feel as strongly about multiple filters as they do multiple heaters .
You definitely don't want to hear about my crazy overkill sumps and their 3000 gph return pumps!
But yeah for many a single AC70 should be fine on a 40g and for most would be an improvement I dare say.