My fiance and I have 2 male dogs, one of which is nuetered. We have had them since they were puppies, although we did acquire the 2nd puppy (his littermate) 6 weeks later simply because the original owner had severe pet allergies. Recently we stepped out of the house for a few hours and when we returned there was blood everywhere. After cleaning them up and having them checked out we learned that their wounds were superficial, but what would cause them to do this after being with us for 16 months.
It sounds like your dogs are reaching the age of maturity. They are establishing their rank with each other. They not only will test each other but will/could test you too. What kind of dogs are they ? Some breeds are naturally dog aggressive and cannot live with another dog or animal in the home. Also, there is a higher incidence of fighting amongst same sex animals. Either way ,they should not be left alone together unsupervised or you may wind up coming home to 1 or 2 dead dogs. Neutering the one who is not already may help some with this. Good luck.
My guess is, they are deciding which of them is boss. One has to be. I suggest you observe them together, see which one keeps winning (i.e., getting the good toy, getting to come over to you first, getting to pin the other on its back). That one is the dominant one. To keep the peace, be sure THAT one understands that you are boss over it. But, I'd let them establish their own pack.