Assuming the two DisplayObjetcs are next to each other on stage, they have a common parent, so a way to "target" obj2 from obj1 would be:
this.rotation = this.parent.getChildByName("obj2").y / 10;
In other words, unless you set up your own references to other DisplayObjetcs like in Pan's answer, you can reference them through their position in the hierarchal display list.