You know, my two still bicker probably twice a week and its been two years. I think its just like any other siblings. They grate on each others nerves, get jealous of one another and even know what makes the other mad... its actually really interesting to observe. Anyone that says dogs don't experience human emotions hasn't met my dogs!
I would say that they've just really started to play with one another within the past month. They would tug at the same toy in the past and chase one another but weren't really interacting until recently. They've suddenly started acting like puppies bounding around the house like pals lately. So, there is hope!
Just may take more time than you expected.