Boeing Field is constantly operating from both runways at the same time.  The light aircraft traffic on the short runway can co-exist with whatever is underway on the main, long runway.  However, despite the clear ATC instructions, there are occasional when things don’t quite go to plan.  We had a Cirrus and a Grand caravan on approach to the parallel runways.  I am not certain who was at fault, but from my angle, it appeared that the Cirrus was drifting off towards the wrong runway.  It corrected its path but not before the pilot of the Grand Caravan decided that things were not looking good and went around.  It didn’t take them long to get back around the pattern and the second approach was incident free.  I don’t know whether the controllers ended up talking to either crew or not.