two-a-days are terrific - I've used them in the past and am working up to them again. However, when using two-a-days, nutrition because even more paramount than when lifting once per day. Two sessions will send your metabolism sky high - pre, intra and post-workout with carbs, amino acids and protein is a must as well as a large amount of vitamins, minerals and antioxidants throughout the day (i.e. fruits, veggies, healthy fats, etc.).
If you're not eating enough, you'll burn out - quick!
However, if your are, you'll lose fat and gain muscle - quick!
Also, lots of sleep is a big help and every two to three weeks reduce your frequency to once-a-day for a week, then resume two-a-days. The change prevents overtraining and gives opportunity for recovery.