Just tried it with a cool negg with my pet and hp went 10-16 and level jumped by 4 same as the duping method. Just make sure hunger level is low. Please confirm if you try it and it works.
Edit- I should've titled it differently. It may do more than double feed with a single item if it really does work. Anyone else try it and have it work? I'm sure it would work with cupcakes if anyone isn't trying to waste a negg