I do 2 a days usually a few times a week.  Be careful doing so because you're really not going to gain anything unless you're already maximized out elsewhere.  
A few thoughts:
- If you're doing both cardio, and weight training, but at separate times, you likely will not see significant muscle gain since the two (and your dieting) will offset.  You'll likely overall lose weight provided you keep a calorie deficit
- If doing a two a day affects your workout the next day, then you shouldn't do it.  If you have good energy the next day and it doesn't affect your sleep, then it may work for you.
- Make sure not to hit the same muscle groups if you decide to weight train in both sessions.  I often will do a "body-builder" style workout targeting only a few muscle groups during the day, and then in the evening will do olympic lifting / crossfit to round it out and get some cardio
- Doing cardio in the morning will usually increase your metabolic rate through most of the day, so on the whole you should have a higher resting calorie intake