Twice a day is fine if you're working two muscles groups liek doing back in the am and bis' in the pm, etc. If you love the gym and won't get burned out quickly bro by all means! BUT, if you're looking just to increase over all volume of workouts, try doing each body part twice a week..some guys, no clue as to why, do two a day and TWICE A WEEK and end up doing their whole bodies 4 times a week. I mean get real. Sometimes less is more. Try it for a few months. If it doesn't work then fuck it, at least by then you'll know. This sport is all about trial and error. I would rather try and know than never do and always wonder.