Its hard for me to believe that you working as hard as possible, so Im thinking its not that you hit a plateau but more that your not working to max. This is also evident because your targeting one muscle group for a whole workout, which has to be either a VERY BORING workout or real short workout. Unless your sleeping inbetween workouts then your not helping yourself in any way. Your body doesnt fully recover until you sleep, and then you cant work chest without working your tricep muscles. So theres no way your working your tris to full capacity in the evening anyway. If your trying to bulk up then your gonna wanna mix up between hypertrophy and strength training, and if your trying to cut up, then your gonna wanna do some cardio and toning. You can still do your two-a-days, but your gonna wanna try a different routine. I would try:
Mon - Legs/Triceps/Abs(night) {if you do two-a-days wait till night to do abs}
Tue - Back/Traps/Shoulder
Wed - Chest/Bicep/Abs(night)
There really no reason to give your Shoulder, Biceps, and or Triceps there own workout unless your actually about to do a competetion. Whenever you work your large muscle groups, you have to work your smaller ones, so in essential your day workouts were both small and large group muscle exercises. You cant work your back without using your bi, you cant do chest without doing triceps or shoulders...