You Fool.

One thing of note is that the two armies have the same number of soldiers. This is because the twins have access to each other's memories. They can prepare for each other very well because they know what the other is planning. 

They can't kill each other themselves because of their synched memories and thoughts. It is why they had to have helpers to kill the other twin. This is not a normal fight at all. The immortals are trying to kill the twins instead of each other. 

It is more of a game. The twins are kings that are to be protected. They don't direct their soldiers at all so that the other twin won't know their tactics. The soldiers have to simultaneously protect their king while trying to kill the other king. 

In a way, this is a fight of luck. The lucky one will have helpers that will kill the other twin first. Or in case someone chooses to interfere and kill one twin first.