gistfile1.txt

MyApp.aSimpleController= SC.ObjectController.create({
  contentBinding : SC.Binding.single('MyApp.anArrayController.selection')
});

MyApp.anotherArrayController= SC.ArrayController.create({
  abcBinding: 'MyApp.aSimpleController.content',
  xyzBinding: SC.Binding.oneWay('MyApp.anotherObjectController.content'),
  observeMe: function(){
    var abc= this.get('abc');
    var xyz= this.get('xyz');
    if (abc && xyz ){
      console.log('abc: '+abc); 
      console.log('xyz: '+xyz);
      console.log('abc should be: '+ MyApp.anArrayController.get('selection').firstObject);
    }
    
  }.observes('abc', 'xyz')
});
'abc' seems to lag behind.
In some situations when abc should be undefined, it is set to the previous value;
output:
abc: MyApp.bla bla
xyz: MyApp.bla bla
abc should be: MyApp.bla bla
//selection set is changed to empty
abc: MyApp.bla bla
xyz: MyApp.bla bla
abc should be: undefined