i always go for overfiltration and idk the numbers on either of these filters but if the 70s add up to over what you need for a 55 then thats perfect 2 filters shouldnt funk up too much either way
having two AC70's on a tank won't do you any good if they both don't restart after a power flicker in the middle of the night. trust me I know from experience haha.
however, if you are diligent in cleaning the AC70's quite often (more often than I was -perhaps weekly??) you probably can avoid this issue altogether. but its a lot of messing around to do so.
you can leave the AC110 impeller chambers untouched for well over a year and they still go on their own if you unplug/plug them back in..
I am not looking to start a big debate over this, but it is an issue with any size AC other than the 110 that should be considered. I don't use any smaller size AC's anymore to avoid this altogether. I used to have to have neighbours on 'filter watch' when I was on vacation..lol. had to show them how to pull up the U-tube, flick the impeller to get it going, put the U-tube back down etc. It was a pain in the ass. Now, its a non issue.