The key is to set npc_aggro in npc_types to 1. Then, set the two mobs on opposing factions. npc_aggro should only be set on the mobs you wish to be involved, and nothing else because as stated above it will cause horrid lag, even if they do not aggro other npcs. But you can certainly have a few different groups duking it out, so two mobs will not cause any lag at all.
NPCs have to check their faction and aggro radius all the time anyway, so that point in this case is not a factor.
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