I suggest allowing players to equip each Zombie pair with one hat for each individual zombie (and equivalent for Gnomes and Kobolds). If a player equips both partners with the same hat, it should still only count as only one copy being used.
(This might not be too smart if there was a need to tell which individual ½-slot characters belong together, but currently the only thing tying e.g. two Kobold partners together is that they share exp. and thus level up simultaneously.)