mouseOut and mouseOver should work on all series
Currently mouseOut and mouseOver events are triggered only for the highest (in layers) series when multiple series are drawn on the same chart. They should be triggered for any. (The reason is because probably tracker captures all mouseOut and mouseOver events for highest layer.)
This actually is more like a bug report, but I am not able to reproduce it. See http://jsfiddle.net/highcharts/bn2rV/, where both series trigger these events.