It may be a good idea to introduce more plants - fake or real as long as they provide alot of coverage - maybe a grass type. - more barriers in tanks prevent aggression and also reduce the possibility of them being around each other - and therefor mistaking each other for food.
The damage looks pretty clean cut, keep an eye on it to make sure it doesn't grow fungus - keep the temps low and the water at optimal quality. It's only a small amount of damage, I think you can expect to see re-growth in a month or two
Have they got a hide between them? or maybe more? I think one hide each will do them well, axolotls only tend to ambush eat so if there's a place to hide away (which they love anyway) then there's less chance again of them being mistaken for food.
The world would be so much better if axolotls could see better +_+