Okay, your terminology confused me a lot here... what you want to do is draw battlers on the battle screen, however have them have the right z-level in order to not cover the skill and item windows when they come up... correct?
In that case, the code you posted has nothing to do with it :p Not to mention it's a little ineffective... with editing it just a little, you could easily draw as many people as you want, not only two...
Either way, what you need to do to solve your problem is set the z-level. You do this as follows:
Now, I don't know what the skill window's z-level is, and probably you don't know either. Even if anyone would, it's smarter to draw it relative to it, in case you'll change the skill window's z-level later on. That's done best in Scene_Battle, where you can reference both objects easily, done like that:
@battler.z = @skill_window.z - 1
Now, this only includes the skill window. Normally you can assume that the skill and item window have the same z-level... but with increasing numbers of cheap scripts around the community, you never know... so, let's make sure:
@battler.z = (@skill_window.z - @item_window.z).abs - 1