I used to run two TetraFin40 HOB
's on my 55g tank.
I switched to a Fluval405 canister.
They both do their jobs just fine, but I find the canister to be MUCH quieter if that's any concern. (Especially as the water level drops a bit.)
The canister is also easier long term... the bio-media lasts more-or-less forever... the filter media can be pretty much whatever you want, (floss, sponge, charcoal, nothing... ) beats having to buy manufactured "cartridges" every time you want to replace the filter media. (IMO
As for circulation, more is generally better
... though I've never had an issue with "dead spots" in any of my tanks... each with only one filter and no powerheads.
's will circulate a little more, if that's still a concern for you... but the REAL benefit is having a backup already in place if one dies! (Clogged intake, broken impeller, miscellaneous failure.)