Dog Socialization

I have 2 dogs... one is a six month old Pug puppy, the other is a 2 year old cocker spaniel. We brought the spaniel into our home about a month ago to help keep the puppy occupied and help with biting tendencies.

They generally seem to get along very well, and don't even seem to have the usual territorial tendencies. They'll eat out of the same bowl at the same time, both sleep in the same kennel, etc. However, when it comes to bones, they seem fiercely competitive. The spaniel will steal the bone from the pug. The pug will bark his head off at the spaniel. Even more curious is that this behaviour seems to continue regardless of how many bones are in play... if there are 4 bones on the floor, they'll be fighting over the 1 in the mouth of the other dog. Anyone know why this is and how I can train them for a little more domestic tranquility?