Basically Zom Com is a Zombie Comedy. So think of comedy with an element of fear. For this I would imagine 2 zombies who falls in love. Like all relationships they have their ups and downs. The kind of things they would argue about are what they plan to do to scare someone and how they resolve this. You may want to add something scary so you could make it seem like a Zom Com. This may also add an element of comedy as well.