If one appears to be the aggressor, have him taken to the vet to have a full meidical check up and also a thyroid blood panel run, often low thyroid can affect behaviour.

In multi dog household one thing humans must understand that dogs need to have a pack heiarchy, they cannot all be treated as equals, one will be more dominant than the other and as humans we need to recognize that and support the more alpha dog by paying more attention to that one, feeding him first, giving him the treats or new toys first, allowing him the favourite sleeping spots even if it means making a more submissive dog move from that spot, etc if you favour the more submissive one instead, the alpha one often will attack the submissive one to correct them to teach them there place in the pack and also to show you that they are the alpha dog. The pack order can change with the addition of new dogs or if one becomes sick or old and will be usurped by a younger and stronger dog as the new leader, with some dogs it is not that apparent and others have no desire to be alpha, but when you have one that wants that role it becomes very necessary to support them or constant disputes will break out.