Did you introduce them immediately or gradually? Normally you'd want to put the new dog in a bathroom or some closed room (with food and water of course) and let the old dog get used to the new dog's smell without seeing it. A day or two later you'd introduce them at feeding in the same room but with separate bowls.
If you didn't do that the dog may feel like his territory is compromised, and it's normal for him to lash out.
Try to go out of your way to treat both of them to special things together
. Feed them together, give them treats together, walk them together. And also go out of your way to spend more time with your first dog so that he doesn't feel like the new dog is taking away his attention.