I'm glad to hear they are already scheduled to be taken to the vet!

I know it's already been said, but I'll echo the rest - neutering them will help curb a lot of behavioral problems, including dominance. Someone mentioned that there may be a female in heat nearby, and that's likely to be something that could have triggered them to act the way they are. But neutering is not a "magic behavior wand" - it will help curb, but not completely solve, behavioral problems in all cases. Talking with your vet is the first step in solving this. Make sure there isn't anything else medically going on with either dog, get them neutered, and see if your vet recommends a trainer or behaviorist in your area.

Good luck!