It's an interesting idea. How does the math work on "both armies attack each other at the same time"? Maybe use the average of both the offensive and defensive kill rates?
It's simple when someone ponders about it in an efficient way. When A hits B, and vice versa, with an equal amount of armies, there should be 50% damage to both of them. It rarely happens that both opponents hit each other with the same amount of armies, but it depends on the bigness of the map, of course.
With regard to counting the defensive kill rates, this shouldn't be done in this case due to both the opponents attacking the neutral territory. Intention is what's behind the factor of whether it's defensive or offensive. And in this case, it surely is the latter, no doubt about it.