They both get fed at the same time - and always have. We make them wait for it, and now DS can say "you can have it" and they both run (in different directions) for their food. Then the boxer is locked on the deck so the beagle can't come and take his food (cos then they fight)

It would be a dominance issue - maybe the boxer is standing up for himself more? Beagle has always been the boss..