Im afraid they are having a terotorial dispute- CAE's have a reputation for being very terotorial fish and vicious. They are terotorial enough when they are just having to put up with other fish in the tank but when you have more than one in a tank this just makes them worse.
What you are veiwing is only the beginning of their bad behavior as CAE's can fit into community tanks when very small but quickly become more and more terotorial as they grow.
Your CAE's are at stage one- one is trying to push the other out of its terotory but obviously they are both trapped in the tank together so neither of them get off one anothers terotory.
This behavior will shortly start to turn more vicious as the CAE's start chasing each other around the tank for longer and longer periods of time and becomming more short tempered towards each other and irrational.
They will start to seriously try to kill each other after this stage- other fish are also targets, particually flat bodied fish or basically any fish that cannot get away quicker enough is a major target- the CAE's will suck off their protective slim layers, attack the fish eyes and fins and anything else they can get a grip on.
Despite being primarily vegetarian fish, they will consume any tank mates of theirs so if fish start to go missing its a major warning you should start seriously thinking about rehoming them or getting them a tank of their own unless you just want to have one well-fed CAE left in your tank and thats all.
Personally, i'd rehome them/get them their own tank now before things start getting nasty as they will do. Keeping them very well fed with algae wafers may hold them off each other for a bit but soon enough no amount of food will be able to hold them off at bay from each other.
