A.L.
My son and daughter are 2 years apart and fight constantly too - they also are great friends and love to play with each other. I've talked to lots of other moms and it's the same story. One friend told me "I knew there would be nothing I could ever do to stop their fighting when we went to the beach in florida and they fought over sand."
I separate my kids if they fight - each has to go play alone in their rooms until they decide they can get along. I don't listen to whose fault it was I just tell them if they can't talk things out they need to stay apart. This works great for us.
I know it's frustrating but I think it's pretty normal. Have you tried reading the book "siblings without rivalry"? It has some great suggestions. Good luck.