Once or twice a day is fine. If you are doing resistance training twice per day, then you should try to make sure the sessions are fairly short.
I would just go the gym whenever it is convenient for you. Working out in the morning seems to be most beneficial because of the tendency for Testosterone levels to peak in the morning and the fact that your workout will be followed by a lot of eating. However, if you perorm all your workouts half awake and miserable, then it's not really worth it.