I actually have the best luck putting on lean muscle when I train each muscle twice a week as well. I hold each workout down to about and hour/hour and 15. When I am trying to cut fat though, my best results are from working each muscle group once a week. I retain more muscle mass and strength this way over training each muscle group twice a week. I agree with Burning_Inside though, when hitting each muscle twice a week, I have to step up the caloric intake. I don't use AAS for recovery, but creatine usually helps.