Just to keep up with tradition (I always seem to be the voice to contradict the general flow of thought)
With one cave you WILL get fighting between male zebs - not something that I like to see.
Even with 4 caves in the tank our alpha male was always being challenged by the 2nd in command - to the point that he ended up getting quite badly marked.
We sold the alpha male to let him go to a home where he would have an easier life... the 2nd in command then took over the role of alpha and we got a spawn, mainly as he had the female trapped on and off for 2 months the original alpha didn't get chance as he was always getting challanged by the other male...
What to do with the tank?
The idea is to force an alpha to emerge. Yes there will be fighting, which is why I said to keep an eye on them 
It is important to provide lots of hiding places if this approach is to be taken, and once the alpha is established an additional cave can be put in the tank, away from the original cave where the alpha is residing.
On the other hand, if you have more caves than males you'll have the risk of cave jumping, and no spawns as a result.
If you have two males that never seem to be able to settle the dispute, like you did, then the best idea is to separate them, like you did.
It is important to provide lots of hiding places if this approach is to be taken, and once the alpha is established an additional cave can be put in the tank, away from the original cave where the alpha is residing.On the other hand, if you have more caves than males you'll have the risk of cave jumping, and no spawns as a result.
If you have two males that never seem to be able to settle the dispute, like you did, then the best idea is to separate them, like you did.
