I2C between Arduinos without Master
You have to have a master for an I2C bus to work.  Can't you allocate one of the dip switches to define the master?

Edit to be able to remove the downvote 
Nick his answer seems to imply that you do not need masters but that is wrong. 
What Nick is saying is that you can swap masters over time.
But each time there is communication there is a master.
So the statement "You have to have a master for an I2C bus to work" is 100% right.
As I misunderstood this myself I downvoted this answer. I hope this edit will enable me to remove my downvote and add a upvote.