I recon your dog maybe thinks that if the food is given to them on a plate or in a bowl, that it belongs to the other dog..... Like it waits for your other dog to finish and then will lick the bowl at the end....
So you may just have to always chuck it on the ground now... Until the other one comes over again and says "hey what are you doing dumb ass? the food on the ground is mine too!"
then you're really screwed...