There are a lot of ways to do it. I opted not to do a more permanent way - creating a level by affixing a mat with aquarium sealant - and used pvc pipe instead to make a platform, then fixed a plastic mat over it. That way the level is removable, as it adds an extra 24 inches of space in the tank that the axolotls actually use. I was going to take it out if they didn't use it, but they hang out their quite frequently. You can make a second level out of pretty much anything. It just takes some creativity.
I really think it depends. I've never had a problem at feeding times, even with the "more aggressive" andersonis. I only state the ability of using the 20 high if the tank was conditioned for it by adding extra things. If axies can live in a tub, it's not that difficult to make a 20 high tank livable for them.
Again, the only reason I switched from my own 20 high is because my andersoni was more active. I got more axolotls after the switch. I think a lot of people on this site overestimate the foot space needed for axolotls. I see some people keeping only one or two in 40 breeders, and I just don't believe that's necessary. While of course I believe that it'd be nice for him to have a 48" tank to make life for him and his axolotls much easier, I think that with the right conditions and care one can certainly attempt this. With close monitoring, they can change it if anything seems to go wrong or they don't feel as happy with it. It's just an option, and his axolotls aren't going to just up and die from it.
How it plays out depends on how much thought he puts into setting up his tank, and making sure to monitor his axolotls.
P.S. My second level is actually made from irrigation pipe and sprinkler heads.