Tenant Issues with duplex yard sharing

[Ryan Schreiber]

Hello,

I recently had a couple move into the 2nd floor unit of a duplex I own.  The couple on the first floor have been there since before I bought the place and have a lot of stuff in the backyard (plants, grills, patio furniture).  No one who lived there before ever wanted to use the backyard.  Now they are having a dispute with the couple on the 2nd floor about backyard usage.  They want an even split in the backyard.  They have left their dog outside by itself and it has torn up plants.  They just can't seem to get along.  

Now they are threatening to leave.

Any experiences with tenants sharing outdoor spaces?