It is a phase.
My twin boys are 18 years old and just left for college, so forgive me if I don't remember ages exactly!
I do remember bringing them in for their 15-month WBC, and one of them was covered with bite marks. Our dear family doctor took one look at J and said "It looks like B's teeth are coming in nicely!"
Mostly we never, ever left them alone for 5 seconds at that age. They could play nicely beside each other, or they could hit, bite, throw toys, steal toys - in other words, act like young toddlers. I don't remember the exact age, but I'm guessing it was around 2 that I could actually leave them alone together to go to the bathroom by myself (prior to that, I brought one with me and left the other one alone in the living room).
Having two of favorite toys doesn't always help - it seemed my kids could distinguish between apparently identical items, and heaven help us if they got hold of the wrong one!
Hang in there, OP (and everyone else) - it honestly DOES get better! Again, I don't know the exact age, but certainly by the time they were 3 they would play together all day, sharing toys, interacting nicely, and having fun.
Now, as young adults, they are still best friends, and sharing a dorm room at college. I don't know what they come up with to talk to each other about for hours on end, but they do.