As many have mentioned, same sex aggression can usually be fixed by a spay/neuter. However, in some cases it doesn’t do the trick. In this case you are going to have to put in work and desensitize the dogs to one another and be sure they know that YOU are the pack leader and that it’s your den. You merely share it with them. If you are uncomfortable desensitizing the dogs on your own I recommend that you seek professional help with it.
Three easy steps to pack leadership:
1) Schedule your dogs feeding. The pack leader determines when the pack eats in nature. Not to mention: What comes in on a schedule comes out on a schedule.
2) Never let your dogs go through your front door (or any other door that leads in or out of the building before you). This shows your dogs that you are going to look out for them and make sure the area is safe.
3) Take your dogs spot. This shows both your dogs that YOU are the pack leader. If they are on your couch, make them move and then sit down in the spot they were in, and then invite them back up. Also, if they are asleep on the ground, wake them up and make them move.
Once you have established yourself as the pack leader, if the aggression continues begin the desensitization process. I recommend a baby gate. One dog gets one side of the house, the other dog gets the other. Gradually let them get used to smelling each other. Once they can smell each other at the gate without growls and barks or hair erection, then let them mingle for 5 minutes, then once they have been okay at 5 minutes for awhile, move it to 15, so on. *WARNING* Desensitization is a very slow process.
Also, were the dogs introduced on neutral territory?