There is no reason not to go with two except for the cost. As noted, it eliminates single point failure from the filtration side of things. But there are other benefits. There is more media space in two AC 20s than a single AC 30. The volume of media to host the needed micro-organisms is normally more important than the flow rate. This changes if one has live plants.
I am a believer in over filtering in terms of media (not flow). One of the benefits of AquaClears is that they have flow control. This allows one to increase the media space by using a larger size modle (rated for s larger tank) but then turning down the flow rate in order to match the needs of the smaller tank. AC states that the lowest flow rate on one of their hang-ons will be about 1/3 of the full flow rate.
Having two filters should not be much more work. Because of the increased media volume you will need to clean it less often. The volume/weight of waste a tank produces does not depend on the filtration. The filtration capacity must be able to handle the amount of waste the tank produces.
My personal opinion on what size AC is appropriate disagrees with what Hagen states about ACs. I prefer to use a larger size. For example, I run a pair of AC 20s on a 15 gal tank. I have a 20 and a 30 on both a 25 and a 29 gallon tank. (All of these are planted.) As a rule of thumb, the only tanks on which I use a single filter are 10 gallons or smaller. I tend to do the same with heaters for similar reasons.
I should add that I am able to afford the methods I use. But then I am pretty old and have managed to reach a point in my life where I am able to do so. I should add that it took me a lot of decades to reach this point. However, along the way I did learn that, in the long run, the cheapest solutions tend to cost the most up front.